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Architectural DNA-binding proteins (ADBPs) are abundant constituents of eukaryotic or bacterial chromosomes that bind DNA promiscuously and function in diverse DNA reactions. They generate large conformational changes in DNA upon binding yet can slide along DNA when searching for functional binding sites. Here we investigate the mechanism by which ADBPs diffuse on DNA by single-molecule analyses of mutant proteins rationally chosen to distinguish between rotation-coupled diffusion and DNA surface sliding after transient unbinding from the groove(s). The properties of yeast Nhp6A mutant proteins, combined with molecular dynamics simulations, suggest Nhp6A switches between two binding modes: a static state, in which the HMGB domain is bound within the minor groove with the DNA highly bent, and a mobile state, where the protein is traveling along the DNA surface by means of its flexible N-terminal basic arm. The behaviors of Fis mutants, a bacterial nucleoid-associated helix-turn-helix dimer, are best explained by mobile proteins unbinding from the major groove and diffusing along the DNA surface. Nhp6A, Fis, and bacterial HU are all near exclusively associated with the chromosome, as packaged within the bacterial nucleoid, and can be modeled by three diffusion modes where HU exhibits the fastest and Fis the slowest diffusion.

DNA binding proteins rapidly locate their specific DNA targets through a combination of 3D and 1D diffusion mechanisms, with the 1D search involving bidirectional sliding along DNA. However, even in nucleosome-free regions, chromosomes are highly decorated with associated proteins that may block sliding. Here we investigate the ability of the abundant chromatin-associated HMGB protein Nhp6A from Saccharomyces cerevisiae to travel along DNA in the presence of other architectural DNA binding proteins using single-molecule fluorescence microscopy. We observed that 1D diffusion by Nhp6A molecules is retarded by increasing densities of the bacterial proteins Fis and HU and by Nhp6A, indicating these structurally diverse proteins impede Nhp6A mobility on DNA. However, the average travel distances were larger than the average distances between neighboring proteins, implying Nhp6A is able to bypass each of these obstacles. Together with molecular dynamics simulations, our analyses suggest two binding modes: mobile molecules that can bypass barriers as they seek out DNA targets, and near stationary molecules that are associated with neighboring proteins or preferred DNA structures. The ability of mobile Nhp6A molecules to bypass different obstacles on DNA suggests they do not block 1D searches by other DNA binding proteins.

Bacteriophage serine integrases catalyze highly specific recombination reactions between defined DNA segments called att sites. These reactions are reversible depending upon the presence of a second phage-encoded directionality factor. The bipartite C-terminal DNA binding region of integrases includes a recombinase domain (RD) connected to a zinc-binding domain (ZD), which contains a long flexible coiled-coil (CC) motif that extends away from the bound DNA. We directly show that the identities of the phage A118 integrase att sites are specified by the DNA spacing between the RD and ZD DNA recognition determinants, which in turn, directs the relative trajectories of the CC motifs on each subunit of the att -bound integrase dimer. Recombination between compatible dimer-bound att sites requires minimal length CC motifs and 14 residues surrounding the tip where pairing of CC motifs between synapsing dimers occurs. Our alanine-scanning data suggests that molecular interactions between CC motif tips may differ in integrative ( attP x attB ) and excisive ( attL x attR ) recombination reactions. We identify mutations in 5 residues within the integrase oligomerization helix that control the remodeling of dimers into tetramers during synaptic complex formation. Whereas most of these gain-of-function mutants still require the CC motifs for synapsis, one mutant efficiently, but indiscriminantly, forms synaptic complexes without the CC motifs. However, the CC motifs are still required for recombination, suggesting a function for the CC motifs after initial assembly of the integrase synaptic tetramer. Importance The robust and exquisitely-regulated site-specific recombination reactions promoted by serine integrases are integral to the life cycle of temperate bacteriophage, and in the case of the A118 prophage, are an important virulence factor by Listeria monocytogenes . The properties of these recombinases have led to their repurposing into tools for genetic engineering and synthetic biology. In this report, we identify determinants regulating synaptic complex formation between correct DNA sites, including the DNA architecture responsible for specifying the identity of recombination sites, features of the unique coiled-coil structure on the integrase that are required to initiate synapsis, and amino acid residues on the integrase oligomerization helix that control the remodeling of synapsing dimers into a tetramer active for DNA strand exchange.

Localized arrays of proteins cooperatively assemble onto chromosomes to control DNA activity in many contexts. Binding cooperativity is often mediated by specific protein–protein interactions, but cooperativity through DNA structure is becoming increasingly recognized as an additional mechanism. During the site-specific DNA recombination reaction that excises phage λ from the chromosome, the bacterial DNA architectural protein Fis recruits multiple λ-encoded Xis proteins to the attR recombination site. Here, we report X-ray crystal structures of DNA complexes containing Fis + Xis, which show little, if any, contacts between the two proteins. Comparisons with structures of DNA complexes containing only Fis or Xis, together with mutant protein and DNA binding studies, support a mechanism for cooperative protein binding solely by DNA allostery. Fis binding both molds the minor groove to potentiate insertion of the Xis β-hairpin wing motif and bends the DNA to facilitate Xis-DNA contacts within the major groove. The Fis-structured minor groove shape that is optimized for Xis binding requires a precisely positioned pyrimidine-purine base-pair step, whose location has been shown to modulate minor groove widths in Fis-bound complexes to different DNA targets.

The serine integrase of phage A118 catalyzes integrative recombination between attP on the phage and a specific attB locus on the chromosome of Listeria monocytogenes , but it is unable to promote excisive recombination between the hybrid attL and attR sites found on the integrated prophage without assistance by a recombination directionality factor (RDF). We have identified and characterized the phage-encoded RDF Gp44, which activates the A118 integrase for excision and inhibits integration. Gp44 binds to the C-terminal DNA binding domain of integrase, and we have localized the primary binding site to be within the mobile coiled-coil (CC) motif but distinct from the distal tip of the CC that is required for recombination. This interaction is sufficient to inhibit integration, but a second interaction involving the N-terminal end of Gp44 is also required to activate excision. We provide evidence that these two contacts modulate the trajectory of the CC motifs as they extend out from the integrase core in a manner dependent upon the identities of the four att sites. Our results support a model whereby Gp44 shapes the Int-bound complexes to control which att sites can synapse and recombine. IMPORTANCE Serine integrases mediate directional recombination between bacteriophage and bacterial chromosomes. These highly regulated site-specific recombination reactions are integral to the life cycle of temperate phage and, in the case of Listeria monocytogenes lysogenized by A118 family phage, are an essential virulence determinant. Serine integrases are also utilized as tools for genetic engineering and synthetic biology because of their exquisite unidirectional control of the DNA exchange reaction. Here, we identify and characterize the recombination directionality factor (RDF) that activates excision and inhibits integration reactions by the phage A118 integrase. We provide evidence that the A118 RDF binds to and modulates the trajectory of the long coiled-coil motif that extends from the large carboxyl-terminal DNA binding domain and is postulated to control the early steps of recombination site synapsis.

The binding of transcription factors (TFs) to DNA controls most aspects of cellular function, making the understanding of their binding kinetics imperative. The standard description of bimolecular interactions posits TF off-rates are independent of TF concentration in solution. However, recent observations have revealed that proteins in solution can accelerate the dissociation of DNA-bound proteins. To study the molecular basis of facilitated dissociation (FD), we have used single-molecule imaging to measure dissociation kinetics of Fis, a key E. coli TF and major bacterial nucleoid protein, from single dsDNA binding sites. We observe a strong FD effect characterized by an exchange rate ∽1 × 104 M-1s-1, establishing that FD of Fis occurs at the single-binding-site level, and we find that the off-rate saturates at large Fis concentrations in solution. While spontaneous (i.e., competitor-free) dissociation shows a strong salt dependence, we find that facilitated dissociation depends only weakly on salt. These results are quantitatively explained by a model in which partially dissociated bound proteins are susceptible to invasion by competitor proteins in solution. We also report FD of NHP6A, a yeast TF whose structure differs significantly from Fis. We further perform molecular dynamics simulations, which indicate that FD can occur for molecules that interact far more weakly than those we have studied. Taken together, our results indicate that FD is a general mechanism assisting in the local removal of TFs from their binding sites and does not necessarily require cooperativity, clustering, or binding site overlap.SIGNIFICANCE STATEMENTTranscription factors (TFs) control biological processes by binding and unbinding to DNA. Therefore it is crucial to understand the mechanisms that affect TF binding kinetics. Recent studies challenge the standard picture of TF binding kinetics by demonstrating cases of proteins in solution accelerating TF dissociation rates through a facilitated dissociation (FD) process. Our study shows that FD can occur at the level of single binding sites, without the action of large protein clusters or long DNA segments. Our results quantitatively support a model of FD in which competitor proteins invade partially dissociated states of DNA-bound TFs. FD is expected to be a general mechanism for modulating gene expression by altering the occupancy of TFs on the genome.Author ContributionsRamsey I. Kamardesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paperEdward J. Banigandesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paperAykut Erbasdesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paperRebecca D. Giuntolidesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paperMonica Olvera de la Cruzdesigned research, performed research, wrote the paperReid C. Johnsondesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paperJohn F. Markodesigned research, performed research, contributed new reagents/analytic tools, analyzed data, wrote the paper

Off-rates of proteins from the DNA double helix are widely considered to be dependent only on the interactions inside the initially bound protein-DNA complex and not on the concentration of nearby molecules. However, a number of recent single-DNA experiments have shown off-rates that depend on solution protein concentration, or “facilitated dissociation.” Here, we demonstrate that this effect occurs for the major Escherichia coli nucleoid protein Fis on isolated bacterial chromosomes. We isolated E. coli nucleoids and showed that dissociation of green fluorescent protein (GFP)-Fis is controlled by solution Fis concentration and exhibits an “exchange” rate constant ( k exch ) of ≈10 4 M −1 s −1 , comparable to the rate observed in single-DNA experiments. We also show that this effect is strongly salt dependent. Our results establish that facilitated dissociation can be observed in vitro on chromosomes assembled in vivo . IMPORTANCE Bacteria are important model systems for the study of gene regulation and chromosome dynamics, both of which fundamentally depend on the kinetics of binding and unbinding of proteins to DNA. In experiments on isolated E. coli chromosomes, this study showed that the prolific transcription factor and chromosome packaging protein Fis displays a strong dependence of its off-rate from the bacterial chromosome on Fis concentration, similar to that observed in in vitro experiments. Therefore, the free cellular DNA-binding protein concentration can strongly affect lifetimes of proteins bound to the chromosome and must be taken into account in quantitative considerations of gene regulation. These results have particularly profound implications for transcription factors where DNA binding lifetimes can be a critical determinant of regulatory function.

DNA strand exchange by serine recombinases has been proposed to occur by a large-scale rotation of halves of the recombinase tetramer. Here we provide the first direct physical evidence for the subunit rotation mechanism for the Hin serine invertase. Single-DNA looping assays using an activated mutant (Hin-H107Y) reveal specific synapses between two hix sites. Two-DNA “braiding” experiments, where separate DNA molecules carrying a single hix are interwound, show that Hin-H107Y cleaves both hix sites and mediates multi-step rotational relaxation of the interwinding. The variable numbers of rotations in the DNA braid experiments are in accord with data from bulk experiments that follow DNA topological changes accompanying recombination by the hyperactive enzyme. The relatively slow Hin rotation rates, combined with pauses, indicate considerable rotary friction between synapsed subunit pairs. A rotational pausing mechanism intrinsic to serine recombinases is likely to be crucial for DNA ligation and for preventing deleterious DNA rearrangements.

The abundant Fis nucleoid protein selectively binds poorly related DNA sequences with high affinities to regulate diverse DNA reactions. Fis binds DNA primarily through DNA backbone contacts and selects target sites by reading conformational properties of DNA sequences, most prominently intrinsic minor groove widths. High-affinity binding requires Fis-stabilized DNA conformational changes that vary depending on DNA sequence. In order to better understand the molecular basis for high affinity site recognition, we analyzed the effects of DNA sequence within and flanking the core Fis binding site on binding affinity and DNA structure. X-ray crystal structures of Fis-DNA complexes containing variable sequences in the noncontacted center of the binding site or variations within the major groove interfaces show that the DNA can adapt to the Fis dimer surface asymmetrically. We show that the presence and position of pyrimidine-purine base steps within the major groove interfaces affect both local DNA bending and minor groove compression to modulate affinities and lifetimes of Fis-DNA complexes. Sequences flanking the core binding site also modulate complex affinities, lifetimes, and the degree of local and global Fis-induced DNA bending. In particular, a G immediately upstream of the 15 bp core sequence inhibits binding and bending, and A-tracts within the flanking base pairs increase both complex lifetimes and global DNA curvatures. Taken together, our observations support a revised DNA motif specifying high-affinity Fis binding and highlight the range of conformations that Fis-bound DNA can adopt. The affinities and DNA conformations of individual Fis-DNA complexes are likely to be tailored to their context-specific biological functions.

The multistep kinetics through which DNA-binding proteins bind their targets are heavily studied, but relatively little attention has been paid to proteins leaving the double helix. Using single-DNA stretching and fluorescence detection, we find that sequence-neutral DNA-binding proteins Fis, HU and NHP6A readily exchange with themselves and with each other. In experiments focused on the Escherichia coli nucleoid-associated protein Fis, only a small fraction of protein bound to DNA spontaneously dissociates into protein-free solution. However, if Fis is present in solution, we find that a concentration-dependent exchange reaction occurs which turns over the bound protein, with a rate of k(exch) = 6 × 10(4) M(-1)s(-1). The bacterial DNA-binding protein HU and the yeast HMGB protein NHP6A display the same phenomenon of protein in solution accelerating dissociation of previously bound labeled proteins as exchange occurs. Thus, solvated proteins can play a key role in facilitating removal and renewal of proteins bound to the double helix, an effect that likely plays a major role in promoting the turnover of proteins bound to DNA in vivo and, therefore, in controlling the dynamics of gene regulation.

The Saccharomyces cerevisiae protein Nhp6A is a model for the abundant and multifunctional high-mobility group B (HMGB) family of chromatin-associated proteins. Nhp6A binds DNA in vitro without sequence specificity and bends DNA sharply, but its role in chromosome biology is poorly understood. We show by whole-genome chromatin immunoprecipitation (ChIP) and high-resolution whole-genome tiling arrays (ChIP–chip) that Nhp6A is localized to specific regions of chromosomes that include ∼23% of RNA polymerase II promoters. Nhp6A binding functions to stabilize nucleosomes, particularly at the transcription start site of these genes. Both genomic binding and transcript expression studies point to functionally related groups of genes that are bound specifically by Nhp6A and whose transcription is altered by the absence of Nhp6. Genomic analyses of Nhp6A mutants specifically defective in DNA bending reveal a critical role of DNA bending for stabilizing chromatin and coregulation of transcription but not for targeted binding by Nhp6A. We conclude that the chromatin environment, not DNA sequence recognition, localizes Nhp6A binding, and that Nhp6A stabilizes chromatin structure and coregulates transcription.

HU is one of the most abundant proteins in bacterial chromosomes and participates in nucleoid compaction and gene regulation. We report experiments using DNA stretching that study the dependence of DNA condensation by HU on force, salt and HU concentration. Previous experiments at sub-physiological salt levels revealed that low concentrations of HU could compact DNA, whereas larger HU concentrations formed a DNA-stiffening complex. Here we report that this bimodal binding behavior depends sensitively on salt concentration. Only the compaction mode was observed for 150 mM and higher NaCl levels, i.e. for physiological salt concentrations. Similar results were obtained for the more physiological salt K-glutamate. Real-time studies of dissociation kinetics revealed that HU unbound slowly (minutes to hours under the conditions studied) but completely for salt concentrations at or above 100 mM NaCl; the lifetime of HU complexes was observed to increase with the HU concentration at which the complexes were formed, and to decrease with salt concentration. Higher salt levels of 300 mM NaCl completely eliminated observable HU binding to DNA. Finally, we observed that the dissociation kinetics depend on force applied to the DNA: increased applied force in the sub-piconewton range accelerates dissociation, suggesting a mechanism for DNA tension to regulate chromosome structure and gene expression.

The bacterial nucleoid-associated protein Fis regulates diverse reactions by bending DNA and through DNA-dependent interactions with other control proteins and enzymes. In addition to dynamic nonspecific binding to DNA, Fis forms stable complexes with DNA segments that share little sequence conservation. Here we report the first crystal structures of Fis bound to high- and low-affinity 27-base-pair DNA sites. These 11 structures reveal that Fis selects targets primarily through indirect recognition mechanisms involving the shape of the minor groove and sequence-dependent induced fits over adjacent major groove interfaces. The DNA shows an overall curvature of approximately 65 degrees , and the unprecedented close spacing between helix-turn-helix motifs present in the apodimer is accommodated by severe compression of the central minor groove. In silico DNA structure models show that only the roll, twist, and slide parameters are sufficient to reproduce the changes in minor groove widths and recreate the curved Fis-bound DNA structure. Models based on naked DNA structures suggest that Fis initially selects DNA targets with intrinsically narrow minor grooves using the separation between helix-turn-helix motifs in the Fis dimer as a ruler. Then Fis further compresses the minor groove and bends the DNA to generate the bound structure.

The phage-encoded Xis protein is the major determinant controlling the direction of recombination in phage lambda. Xis is a winged-helix DNA binding protein that cooperatively binds to the attR recombination site to generate a curved microfilament, which promotes assembly of the excisive intasome but inhibits formation of an integrative intasome. We find that lambda synthesizes surprisingly high levels of Xis immediately upon prophage induction when excision rates are maximal. However, because of its low sequence-specific binding activity, exemplified by a 1.9 Å co-crystal structure of a nonspecifically bound DNA complex, Xis is relatively ineffective at promoting excision in vivo in the absence of the host Fis protein. Fis binds to a segment in attR that almost entirely overlaps one of the Xis binding sites. Instead of sterically excluding Xis binding from this site, as has been previously believed, we show that Fis enhances binding of all three Xis protomers to generate the microfilament. A specific Fis-Xis interface is supported by the effects of mutations within each protein, and relaxed, but not completely sequence-neutral, binding by the central Xis protomer is supported by the effects of DNA mutations. We present a structural model for the 50 bp curved Fis-Xis cooperative complex that is assembled between the arm and Holliday junction Int binding sites whose trajectory places constraints on models for the excisive intasome structure.

The DNA architectural protein Xis regulates the construction of higher-order nucleoprotein intasomes that integrate and excise the genome of phage lambda from the Escherichia coli chromosome. Xis modulates the directionality of site-specific recombination by stimulating phage excision 10 6 -fold, while simultaneously inhibiting phage reintegration. Control is exerted by cooperatively assembling onto a ≈35-bp DNA regulatory element, which it distorts to preferentially stabilize an excisive intasome. Here, we report the 2.6-Å crystal structure of the complex between three cooperatively bound Xis proteins and a 33-bp DNA containing the regulatory element. Xis binds DNA in a head-to-tail orientation to generate a micronucleoprotein filament. Although each protomer is anchored to the duplex by a similar set of nonbase specific contacts, malleable protein–DNA interactions enable binding to sites that differ in nucleotide sequence. Proteins at the ends of the duplex sequence specifically recognize similar binding sites and participate in cooperative binding via protein–protein interactions with a bridging Xis protomer that is bound in a less specific manner. Formation of this polymer introduces ≈72° of curvature into the DNA with slight positive writhe, which functions to connect disparate segments of DNA bridged by integrase within the excisive intasome.

The rate of dissociation of a DNA-protein complex is often considered to be a property of that complex, without dependence on other nearby molecules in solution. We study the kinetics of dissociation of the abundant Escherichia coli nucleoid protein Fis from DNA, using a single-molecule mechanics assay. The rate of Fis dissociation from DNA is strongly dependent on the solution concentration of DNA. The off-rate (k(off)) of Fis from DNA shows an initially linear dependence on solution DNA concentration, characterized by an exchange rate of k(ex)≈9×10(-4) (ng/μl)(-1) s(-1) for 100 mM univalent salt buffer, with a very small off-rate at zero DNA concentration. The off-rate saturates at approximately k(off,max)≈8×10(-3) s(-1) for DNA concentrations above ≈20 ng/μl. This exchange reaction depends mainly on DNA concentration with little dependence on the length of the DNA molecules in solution or on binding affinity, but this does increase with increasing salt concentration. We also show data for the yeast HMGB protein NHP6A showing a similar DNA-concentration-dependent dissociation effect, with faster rates suggesting generally weaker DNA binding by NHP6A relative to Fis. Our results are well described by a model with an intermediate partially dissociated state where the protein is susceptible to being captured by a second DNA segment, in the manner of "direct transfer" reactions studied for other DNA-binding proteins. This type of dissociation pathway may be important to protein-DNA binding kinetics in vivo where DNA concentrations are large.

Two critical steps controlling serine recombinase activity are the remodeling of dimers into the chemically active synaptic tetramer and the regulation of subunit rotation during DNA exchange. We identify a set of hydrophobic residues within the oligomerization helix that controls these steps by the Hin DNA invertase. Phe105 and Met109 insert into hydrophobic pockets within the catalytic domain of the same subunit to stabilize the inactive dimer conformation. These rotate out of the catalytic domain in the dimer and into the subunit rotation interface of the tetramer. About half of residue 105 and 109 substitutions gain the ability to generate stable synaptic tetramers and/or promote DNA chemistry without activation by the Fis/enhancer element. Phe106 replaces Phe105 in the catalytic domain pocket to stabilize the tetramer conformation. Significantly, many of the residue 105 and 109 substitutions support subunit rotation but impair ligation, implying a defect in rotational pausing at the tetrameric conformer poised for ligation. We propose that a ratchet-like surface involving Phe105, Met109 and Leu112 within the rotation interface functions to gate the subunit rotation reaction. Hydrophobic residues are present in analogous positions in other serine recombinases and likely perform similar functions.

Reversible site-specific DNA inversion reactions are widely distributed in bacteria and their viruses. They control a range of biological reactions that most often involve alterations of molecules on the surface of cells or phage. These programmed DNA rearrangements usually occur at a low frequency, thereby preadapting a small subset of the population to a change in environmental conditions, or in the case of phages, an expanded host range. A dedicated recombinase, sometimes with the aid of additional regulatory or DNA architectural proteins, catalyzes the inversion of DNA. RecA or other components of the general recombination-repair machinery are not involved. This chapter discusses site-specific DNA inversion reactions mediated by the serine recombinase family of enzymes and focuses on the extensively studied serine DNA invertases that are stringently controlled by the Fis-bound enhancer regulatory system. The first section summarizes biological features and general properties of inversion reactions by the Fis/enhancer-dependent serine invertases and the recently described serine DNA invertases in Bacteroides . Mechanistic studies of reactions catalyzed by the Hin and Gin invertases are then explored in more depth, particularly with regards to recent advances in our understanding of the function of the Fis/enhancer regulatory system. These include the steps leading to the formation of the active recombination complex (invertasome) containing the recombinase tetramer and Fis/enhancer element and the process of DNA strand exchange by rotation of synapsed subunit pairs within the invertasome. The role of DNA topological forces that function in concert with the Fis/enhancer controlling element in specifying the overwhelming bias for DNA inversion over deletion and intermolecular recombination is also discussed.

Upon induction of a bacteriophage λ lysogen, a site-specific recombination reaction excises the phage genome from the chromosome of its bacterial host. A critical regulator of this process is the phage-encoded excisionase (Xis) protein, which functions both as a DNA architectural factor and by cooperatively recruiting integrase to an adjacent binding site specifically required for excision. Here we present the three-dimensional structure of Xis and the results of a structure-based mutagenesis study to define the molecular basis of its function. Xis adopts an unusual “winged”-helix motif that is modeled to interact with the major- and minor-grooves of its binding site through a single α-helix and loop structure (“wing”), respectively. The C-terminal tail of Xis, which is required for cooperative binding with integrase, is unstructured in the absence of DNA. We propose that asymmetric bending of DNA by Xis positions its unstructured C-terminal tail for direct contacts with the N-terminal DNA-binding domain of integrase and that an ensuing disordered to ordered transition of the tail may act to stabilize the formation of the tripartite integrase–Xis–DNA complex required for phage excision.

5,6-Dihydrouridine (D) is a modified base found abundantly in the D-loops of tRNA from Archaea, Bacteria, and Eukarya. D is thought to be formed post-transcriptionally by the reduction of uridines in tRNA transcripts. Despite its abundance, no enzymes that catalyze D-formation have been identified. Using comparative genomics and computational methods we have identified members of the cluster of orthologous genes, COG0042, as putative dihydrouridine synthase encoding genes. Escherichia coli contains three COG0042 family members (yjbN, yhdG, and yohI). Strains were created where one, two, or all three of the COG0042 genes were deleted. Purified tRNA samples were investigated from the three single and the three double knockout strains, as well as from the triple deletion strain. The results showed that the COG0042 gene family is responsible for tRNA-dihydrouridine synthase activity in E. coli. They also suggest that the COG0042-encoded family members act site-specifically on the tRNA D-loop and contain non-redundant catalytic functions in vivo.

HMGB1 (also called HMG-1) is a DNA-bending protein that augments the affinity of diverse regulatory proteins for their DNA sites. Previous studies have argued for a specific interaction between HMGB1 and target proteins, which leads to cooperative binding of the complex to DNA. Here we propose a different model that emerged from studying how HMGB1 stimulates enhanceosome formation by the Epstein-Barr viral activator Rta on a target gene, BHLF-1. HMGB1 stimulates binding of individual Rta dimers to multiple sites in the enhancer. DNase I and hydroxyl radical footprinting, electrophoretic mobility shift assays, and immobilized template assays failed to reveal stable binding of HMGB1 within the complex. Furthermore, mutational analysis failed to identify a specific HMGB1 target sequence. The effect of HMGB1 on Rta could be reproduced by individual HMG domains, yeast HMO1, or bacterial HU. These results, combined with the effects of single-amino-acid substitutions within the DNA-binding surface of HMGB1 domain A, argue for a mechanism whereby DNA-binding and bending by HMGB1 stimulate Rta-DNA complex formation in the absence of direct interaction with Rta or a specific HMGB1 target sequence. The data contrast with our analysis of HMGB1 action on another BHLF-1 regulatory protein called ZEBRA. We discuss the two distinct modes of HMGB1 action on a single regulatory region and propose how HMGB1 can function in diverse contexts.

The transcription factor Fis activates the Escherichia coli rRNA promoters rrnB P1 and rrnE P1 by binding to sites centered at -71 and -72, respectively, and interacting with the C-terminal domain of the alpha subunit of RNA polymerase (RNAP alphaCTD). To understand the mechanism of activation by Fis at these promoters, we used oriented alpha-heterodimeric RNAPs and heterodimers of Fis to determine whether one or both subunits of alpha and Fis participate in the alphaCTD-Fis interaction. Our results imply that only one alphaCTD in the alpha dimer and only one activation-proficient subunit in the Fis dimer are required for activation by Fis. A library of alanine substitutions in alpha was used to identify the alphaCTD determinants required for Fis-dependent transcription at rrnB P1 and rrnE P1. We propose that the transcriptional activation region of the promoter-proximal subunit of the Fis dimer interacts with a determinant that includes E273 of one alphaCTD to activate transcription. We further suggest that the Fis contact to alphaCTD results in alphaCTD interactions with DNA that differ somewhat from those that occur at UP elements in the absence of Fis. The accompanying paper shows that the 273 determinant on alphaCTD is also targeted by Fis at the proP P2 promoter where the activator binds overlapping the -35 hexamer. Thus, similar Fis-alphaCTD interactions are used for activation of transcription when the activator is bound at very different positions on the DNA.

This chapter focuses on the major nucleoid-associated proteins and summarizes one's current knowledge of how these proteins contribute to the structure of the nucleoid and function in specific reactions involving the chromosome. The current view is that the Escherichia coli nucleoid is dominated by five different proteins: HU, integration host factor (IHF), H-NS, StpA and Fis under nutrient-rich exponential growth conditions. HU mutant cells display the most severe and varied phenotypes in comparison to strains containing mutations in the other major nucleoid proteins. Supercoiling of chromosomal and plasmid DNA is partially relaxed in mutants lacking HU, supporting an in vivo role for HU in regulating the degree of supercoiling. HU plays several interconnected roles in DNA replication and appears to be involved in several steps involving initiation of chromosomal DNA synthesis, chromosomal partitioning, and cell division. The genes that were differentially expressed were examined for putative IHF binding sites in the 500 bp upstream of the transcription initiation site. The criteria for identifying high-affinity sites were that the sites must have a 12 out of 13 bp match with the core consensus sequence and that at least 10 of the 15 bp 5' (upstream) to the core consensus must be dA-dT base pairs. The study identified 46 candidates that matched the criteria, some of which were documented in other studies. The study also identified seven genes, including ihfA, that were expressed only when IHF was present and eight genes that were expressed only in IHF-deficient cells.

The Saccharomyces cerevisiae non-histone protein 6-A (NHP6A) is a member of the high-mobility group 1/2 protein family that bind and bend DNA of mixed sequence. NHP6A has only one high-mobility group 1/2 DNA binding domain and also requires a 16-amino-acid basic tail at its N-terminus for DNA binding. We show in this report that nuclear accumulation of NHP6A is strictly correlated with its DNA binding properties since only nonhistone protein 6 A-green fluorescent protein chimeras that were competent for DNA binding were localized to the nucleus. Despite the requirement for basic residues within the N-terminal segment for DNA binding and nuclear accumulation, this region does not appear to contain a nuclear localization signal. Moreover, NHP6A does not bind to the yeast nuclear localization signal receptor SRP1 and nuclear targeting of NHP6A does not require the function of the 14 different importins. Unlike histone H2B1 which contains a classical nuclear localization signal, entry of NHP6A into the nucleus was found to be independent of Ran as judged by coexpression of Ran GTPase mutants and was shown to occur at 0 degrees C after a 15-min induction. These unusual properties lead us to suggest that NHP6A entry into the nucleus proceeds by a nonclassical Ran-independent pathway.

Transcription of the P1 promoter of the Escherichia coli proP gene, which encodes a transporter of osmoprotectants, is strongly induced by a shift to hyperosmotic media. Unlike most other osmotically regulated promoters, the induction occurs for a brief period of time, corresponding to the replacement of intracellular K(+) glutamate with osmoprotecting compounds. This burst of proP transcription is correlated with the osmolarity-dependent binding of the cAMP receptor protein CRP to a site within the proP P1 promoter. We show that CRP-cAMP functions as an osmotically sensitive repressor of proP P1 transcription in vitro. Binding of CRP to the proP promoter in vivo is transiently destabilized after a hyperosmotic shift with kinetics that correspond to the derepression of transcription, whereas Fis and Lac repressor binding is not osmotically sensitive. Similar osmotic regulation of proP P1 transcription by the CRP* mutant implies that binding of cAMP is not responsible for the unusual osmotic sensitivity of CRP activity. Osmotic regulation of CRP activity is not limited to proP. Activation of the lac promoter by CRP is also transiently inhibited after an osmotic upshift, as is the binding of CRP to the galdelta4P1 promoter. These findings suggest that CRP functions in certain contexts to regulate gene expression in response to osmotic changes, in addition to its role in catabolite control.

In a recent issue of Molecular Cell, Mouw et al. (2008) report a crystal structure of a serine recombinase bound to a regulatory DNA site in an unexpected synaptic complex configuration, which forms the framework for a new model of the entire 12 subunit, 186 bp deletion complex.

The Hin DNA invertase becomes catalytically activated when assembled in an invertasome complex containing two Fis dimers bound to an enhancer segment. The region of Fis responsible for transactivation of Hin contains a mobile β-hairpin arm that extends from each dimer subunit. We show here that whereas both Fis dimers must be capable of activating Hin, Fis heterodimers that have only one functional activating β-arm are sufficient to form catalytically competent invertasomes. Analysis of homodimer and heterodimer mixes of different Hin mutants suggests that Fis must activate each subunit of the two Hin dimers that participate in catalysis. These experiments also indicate that all four Hin subunits must be coordinately activated prior to initiation of the first chemical step of the reaction and that the process of activation is independent of the catalytic steps of recombination. We propose a molecular model for the invertasome structure that is consistent with current information on protein–DNA structures and the topology of the DNA strands within the recombination complex. In this model, a single Fis activation arm could contact amino acids from both Hin subunits at the dimer interface to induce a conformational change that coordinately positions the active sites close to the scissile phosphodiester bonds.

Serine recombinases are often tightly controlled by elaborate, topologically-defined, nucleoprotein complexes. Hin is a member of the DNA invertase subclass of serine recombinases that are regulated by a remote recombinational enhancer element containing two binding sites for the protein Fis. Two Hin dimers bound to specific recombination sites associate with the Fis-bound enhancer by DNA looping where they are remodeled into a synaptic tetramer competent for DNA chemistry and exchange. Here we show that the flexible beta-hairpin arms of the Fis dimers contact the DNA binding domain of one subunit of each Hin dimer. These contacts sandwich the Hin dimers to promote remodeling into the tetramer. A basic region on the Hin catalytic domain then contacts enhancer DNA to complete assembly of the active Hin tetramer. Our results reveal how the enhancer generates the recombination complex that specifies DNA inversion and regulates DNA exchange by the subunit rotation mechanism. DOI: http://dx.doi.org/10.7554/eLife.01211.001, eLife digest Many processes in biology rely on enzymes that break both the strands in a DNA molecule, then rearrange the strands, and finally join them back together in a new configuration. These recombination reactions can, for example, change the positions of genetic elements such as enhancers and promoters within the DNA molecule and, therefore, influence how a given gene is expressed as a protein. Cells need to be able to control recombination reactions because they can lead to leukemia and lymphomas if they go wrong. The enzymes that catalyze these recombination reactions are called recombinases. One type of recombinase binds to specific sequences of DNA bases and uses an amino acid in the enzyme–usually serine or tyrosine–to break and rejoin the DNA strands. Recombination reactions require the assembly of complexes containing many proteins bound to DNA. Tyrosine recombinases form relatively simple protein-DNA complexes, and these have been studied in detail. Serine recombinases, on the other hand, form more elaborate protein-DNA complexes, and much less is known about these. Now McLean et al. have unraveled the mechanism that a serine recombinase called Hin uses to reverse the direction of a stretch of chromosomal DNA in the bacteria Salmonella enterica. Inverting this stretch of DNA–which contains about 1000 base pairs–changes the position of a gene promoter that is responsible for the production of flagellin, which is the protein that enables the bacterium to move. This is one of the tricks that Salmonella uses to evade the immune system of its host. Previous research has established that four Hin subunits and two copies of a protein called Fis are needed to invert this stretch of DNA: two Hin subunits bind to each of the two hix recombination sites, and the Fis proteins (which are dimers) bind to each end of an enhancer that is located between the hix sites. A protein called HU then causes the DNA to bend and form a loop, and the four Hin subunits and the two Fis dimers all come together at the enhancer to form a structure called the invertasome where the recombination reaction occurs. All four DNA strands at the crossover point are broken as a result of a near simultaneous attack by the catalytic serine amino acids in the Hin subunits. One pair of Hin subunits–and the two DNA strands attached to them–then rotate by 180 degrees around the other pair of Hin subunits. This means that the stretch of DNA between the hix sites is inverted when the DNA strands are rejoined at the end of the reaction. Enhancers often regulate transcription and other reactions from a distance. McLean et al. reveal how an enhancer of a DNA recombination reaction works. The pairs of Hin subunits that initially bind to the DNA are not catalytically active, but when they are brought together by the enhancer and form a tetramer, they become active. Two of the Hin subunits are clamped onto the enhancer by the Fis dimers and by directly interacting with the enhancer DNA, but the other two (and the DNA strands attached to them) are free to rotate within the tetramer. In the Salmonella chromosome the enhancer is located close to one of the hix sites (∼100 base pairs away from it), so the length of the DNA between the enhancer and hix site physically limits the number of Hin subunit rotations to just one. DOI: http://dx.doi.org/10.7554/eLife.01211.002

Background A large subfamily of serine recombinases contains long polypeptide segments appended to the C-terminal end of the conserved catalytic domain. Members of this subfamily often function as phage integrases but also mediate transposition and regulate terminal differentiation processes in eubacteria. Although a few members of this subfamily have been studied in purified in vitro systems, key mechanistic aspects of reactions promoted by these recombinases remain to be determined, particularly with respect to the functions of the large C-terminal domain. Results We have developed and characterized a robust in vitro recombination reaction by the Listeria phage A118 integrase, a member of the subfamily of serine recombinases containing a large C-terminal domain. The reaction occurs in a simple buffered salt solution and exhibits a modest stimulation by divalent cations or spermidine and DNA supercoiling. Recombination with purified A118 integrase is unidirectional, being efficient only between attP and attB DNA sites to either join separate DNA molecules (intermolecular recombination) or to generate deletions or inversions depending on the relative orientation of att sites in cis (intramolecular recombination). The minimal attP site is 50 bp but requires only 44 bp of base sequence information, whereas the minimal attB site is 42 bp and requires 38 bp of base sequence information. DNA exchange occurs between the central 2 bp of attP and attB. Identity between these two base pairs is required for recombination, and they solely determine the orientation of recombination sites. The integrase dimer binds efficiently to full att sites, including the attL and attR integration products, but poorly and differentially to each half-site. The large C-terminal domain can be separated from the N-terminal catalytic by partial proteolysis and mediates non-cooperative DNA binding to att sites. Conclusions The basic properties of the phage A118 integrase reaction and its substrate requirements have been elucidated. A118 integrase thus joins the handful of biochemically characterized serine integrases that are serving as models for mechanistic studies on this important class of recombinases. Information reported here will also be useful in exploiting this recombinase for genetic engineering.

Nonhistone proteins 6A and 6B (NHP6A/B) are nonsequence-specific DNA-binding proteins from Saccharomyces cerevisiae that are related structurally and functionally to the mammalian high mobility group proteins 1 and 2. These DNA architectural proteins distort DNA structure severely and have been shown to promote assembly of specialized recombination complexes. Here we show that the yeast NHP6A/B proteins are required for the induction of a subset of genes transcribed by RNA polymerase II (pol II). Activation of the CUP1, CYC1, GAL1, and DDR2 genes was decreased or abolished completely in the delta nhp6A/B strain. No significant change in basal expression was observed for any of the 10 genes examined. Analysis of chimeric gene constructs localized the regions dependent on NHP6A/B to be primarily at the core promoters, although the GAL1 UAS also requires NHP6A/B for activity. In vitro, NHP6A stimulated transcription by pol II at the GAL1 promoter three- to fivefold above the level of activation by GAL4-VP16 alone. Gel mobility shift assays showed that NHP6A promotes the formation of a complex with TBP and TFIIA at the TATA box that has enhanced affinity for TFIIB.

Synthesis of the Fis nucleoid protein rapidly increases in response to nutrient upshifts, and Fis is one of the most abundant DNA binding proteins in Escherichia coli under nutrient-rich growth conditions. Previous work has shown that control of Fis synthesis occurs at transcription initiation of the dusB-fis operon. We show here that while translation of the dihydrouridine synthase gene dusB is low, unusual mechanisms operate to enable robust translation of fis. At least two RNA sequence elements located within the dusB coding region are responsible for high fis translation. The most important is an AU element centered 35 nucleotides (nt) upstream of the fis AUG, which may function as a binding site for ribosomal protein S1. In addition, a 44-nt segment located upstream of the AU element and predicted to form a stem-loop secondary structure plays a prominent role in enhancing fis translation. On the other hand, mutations close to the AUG, including over a potential Shine-Dalgarno sequence, have little effect on Fis protein levels. The AU element and stem-loop regions are phylogenetically conserved within dusB-fis operons of representative enteric bacteria.

Recent studies on site-specific DNA recombination reactions have provided new information on the structure of the DNA-binding and catalytic domains of the related Hin and γδ resolvase recombinases, respectively, and the interaction of the recombinational enhancer protein Fis with DNA. These advances lay the foundation for detailed structural studies of recombination complexes and for elucidating the enzymology of DNA strand exchange in this family of recombinases.

Determining numbers of proteins bound to large DNAs is important for understanding their chromosomal functions. Protein numbers may be affected by physical factors such as mechanical forces generated in DNA, e.g. by transcription or replication. We performed single-DNA stretching experiments with bacterial nucleoid proteins HU and Fis, verifying that the force–extension measurements were in thermodynamic equilibrium. We, therefore, could use a thermodynamic Maxwell relation to deduce the change of protein number on a single DNA due to varied force. For the binding of both HU and Fis under conditions studied, numbers of bound proteins decreased as force was increased. Our experiments showed that most of the bound HU proteins were driven off the DNA at 6.3 pN for HU concentrations lower than 150 nM; our HU data were fit well by a statistical-mechanical model of protein-induced bending of DNA. In contrast, a significant amount of Fis proteins could not be forced off the DNA at forces up to 12 pN and Fis concentrations up to 20 nM. This thermodynamic approach may be applied to measure changes in numbers of a wide variety of molecules bound to DNA or other polymers. Force-dependent DNA binding by proteins suggests mechano-chemical mechanisms for gene regulation.

This paper describes the crystallization, dehydration and preliminary X-ray data analysis of a complex containing several bacteriophage lambda excisionase (Xis) [Bushman et al. (1984). Cell, 39, 699-706] proteins cooperatively bound to a 33-mer DNA duplex (Xis-DNA(X1-X2)). Xis is expected to recognize this regulatory element in a novel manner by cooperatively binding and distorting multiple head-to-tail orientated DNA-binding sites. Crystals of this complex belonged to space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 107.7, c = 73.5 angstroms, alpha = beta = 90, gamma = 120 degrees. Based on the unit-cell parameters for the asymmetric unit, V(M) is 3.0 A(3) Da(-1), which corresponds to a solvent content of approximately 59%. The approaches used to crystallize the unusually long DNA fragment in the complex and the dehydration technique applied that dramatically improved the diffraction of the crystals from 10 to 2.6 angstroms are discussed.

The excisionase (Xis) protein from bacteriophage λ is the best characterized member of a large family of recombination directionality factors that control integrase-mediated DNA rearrangements. It triggers phage excision by cooperatively binding to sites X1 and X2 within the phage, bending DNA significantly and recruiting the phage-encoded integrase (Int) protein to site P2. We have determined the co-crystal structure of Xis with its X2 DNA-binding site at 1.7 A resolution. Xis forms a unique winged-helix motif that interacts with the major and minor grooves of its binding site using an α-helix and an ordered β-hairpin (wing), respectively. Recognition is achieved through an elaborate water-mediated hydrogen-bonding network at the major groove interface, while the preformed hairpin forms largely non-specific interactions with the minor groove. The structure of the complex provides insights into how Xis recruits Int cooperatively, and suggests a plausible mechanism by which it may distort longer DNA fragments significantly. It reveals a surface on the protein that is likely to mediate Xis–Xis interactions required for its cooperative binding to DNA.

It has been shown that Fis activates transcription of the ribosomal promoter rrnB P1; however, the mechanism by which Fis activates rrnB P1 transcription is not fully understood. Paradoxically, although Fis activates transcription of rrnB P1 in vitro, transcription from the promoter containing Fis sites (as measured from rrnB P1-lacZ fusions) is not reduced in a fis null mutant strain. In this study, we further investigated the mechanism by which Fis activates transcription of the rrnB P1 promoter and the role of Fis in rRNA synthesis and cell growth in Escherichia coli. Like all other stringent promoters investigated so far, open complex of rrnB P1 has been shown to be intrinsically unstable, making open complex stability a potential regulatory step in transcription of this class of promoters. Our results show that Fis acts at this regulatory step by stabilizing the interaction between RNA polymerase and rrnB P1 in the absence of NTPs. Mutational analysis of the Fis protein demonstrates that there is a complete correlation between Fis-mediated transcriptional activation of rrnB P1 and Fis-mediated stabilization of preinitiation complexes of the promoter. Thus, our study indicates that Fis-mediated stabilization of RNA polymerase-rrnB P1 preinitiation complexes, presumably at the open complex step, contributes prominently to transcriptional activation. Furthermore, our in vivo results show that rRNA synthesis from the P1 promoters of several rRNA operons are reduced 2-fold in a fis null mutant compared with the wild type strain, indicating that Fis plays an important role in the establishment of robust rRNA synthesis when E. coli cells are emerging from a growth-arrested phase to a rapid growth phase. Thus, our results resolve an apparent paradox of the role of Fis in vitro and in vivo in the field.

Nucleoid proteins are a group of abundant DNA binding proteins that modulate the structure of the bacterial chromosome. They have been recruited as specific negative and positive regulators of gene transcription and their fluctuating patterns of expression are often exploited to impart an additional level of control with respect to environmental conditions.

Assembly of enhanceosomes requires architectural proteins to facilitate the DNA conformational changes accompanying cooperative binding of activators to a regulatory sequence. The architectural protein HMG-1 has been proposed to bind DNA in a sequence-independent manner, yet, paradoxically, it facilitates specific DNA binding reactions in vitro. To investigate the mechanism of specificity we explored the effect of HMG-1 on binding of the Epstein-Barr virus activator ZEBRA to a natural responsive promoter in vitro. DNase I footprinting, mutagenesis, and electrophoretic mobility shift assay reveal that HMG-1 binds cooperatively with ZEBRA to a specific DNA sequence between two adjacent ZEBRA recognition sites. This binding requires a strict alignment between two adjacent ZEBRA sites and both HMG boxes of HMG-1. Our study provides the first demonstration of sequence-dependent binding by a nonspecific HMG-box protein. We hypothesize how a ubiquitous, nonspecific architectural protein can function in a specific context through the use of rudimentary sequence recognition coupled with cooperativity. The observation that an abundant architectural protein can bind DNA cooperatively and specifically has implications towards understanding HMG-1's role in mediating DNA transactions in a variety of enzymological systems.

The chimeric nuclease Fis-OP has been used to identify novel Fis binding sites. Tethering the chemical nuclease OP-Cu+ to position 73 of the protein with a newly developed longer acetyl-beta-alanylamino spacer has facilitated the localization of two high-affinity Fis binding sequences in a 3 kb pUC19 plasmid. The shorter acetamido linker has allowed the chimeric nuclease to locate two strong Fis binding sites in the 50 kb phage lambda genome. All four sites reside in biologically interesting loci and have been confirmed by gel-retardation and DNase I footprint analyses. A newly discovered site resides in the lac operon of Escherichia coli. The binding of Fis to this site may antagonize repression by the LacI repressor. These studies demonstrate the feasibility of applying chimeric chemical nucleases to the task of identifying functional protein binding sites of biological interest within genomes without any assumption about their sequence preference.

The mammalian high mobility group proteins HMG1 and HMG2 are abundant, chromatin-associated proteins whose cellular function is not known. In this study we show that these proteins can substitute for the prokaryotic DNA-bending protein HU in promoting the assembly of the Hin invertasome, an intermediate structure in Hin-mediated site-specific DNA inversion. Formation of this complex requires the assembly of the Hin recombinase, the Fis protein, and three cis-acting DNA sites, necessitating the looping of intervening DNA segments. Invertasome assembly is strongly stimulated by HU or HMG proteins when one of these segments is shorter than 104 bp. By use of ligase-mediated circularization assays, we demonstrate that HMG1 and HMG2 can bend DNA extremely efficiently, forming circles as small as 66 bp, and even 59-bp circles at high HMG protein concentrations. In both invertasome assembly and circularization assays, substrates active in the presence of HMG1 contain one less helical turn of DNA compared with substrates active in the presence of HU protein. Analysis of different domains of HMG1 generated by partial proteolytic digestion indicate that DNA-binding domain B is sufficient for both bending and invertasome assembly. We suggest that an important biological function of HMG1 and HMG2 is to facilitate cooperative interactions between cis-acting proteins by promoting DNA flexibility. A general role for HMG1 and HMG2 in chromatin structure is also suggested by their ability to wrap DNA duplexes into highly compact forms.

Higher-order nucleoprotein complexes are associated with many biological processes. In bacteria the formation of these macromolecular structures for DNA recombination, replication, and transcription often requires not only the participation of specific enzymes and co-factors, but also a class of DNA-binding proteins collectively known as 'nucleoid-associated' or 'histone-like' proteins. Examples of this class of proteins are HU, Integration Host Factor, H-NS, and Fis. Fis was originally identified as the factor for inversion stimulation of the homologous Hin and Gin site-specific DNA recombinases of Salmonella and phage Mu, respectively. This small, basic, DNA-bending protein has recently been shown to function in many other reactions including phage lambda site-specific recombination, transcriptional activation of rRNA and tRNA operons, repression of its own synthesis, and oriC-directed DNA replication. Cellular concentrations of Fis vary tremendously under different growth conditions which may have important regulatory implications for the physiological role of Fis in these different reactions. The X-ray crystal structure of Fis has been determined and insights into its mode of DNA binding and mechanisms of action in these disparate systems are being made.

A wealth of new information regarding the structure of the synaptic complex, the mechanism of DNA strand exchange, and the role of the recombinational enhancer in promoting DNA inversion has been obtained from a combination of approaches. These include: electron microscopy of reaction intermediates, topological analysis of recombination products, and X-ray crystallography coupled with genetic analysis.

Fis is a small, basic, site-specific DNA-binding protein present in Escherichia coli. A Fis-binding site (F) has been previously identified in the attP recombination site of phage lambda (J. F. Thompson, L. Moitoso de Vargas, C. Koch, R. Kahmann, and A. Landy, Cell 50:901-908, 1987). The present study demonstrates that in the absence of the phage-encoded Xis protein, the binding of Fis to F can stimulate integrative recombination and therefore increase the frequency of lambda lysogeny in vivo. Additionally, Fis exerts a stimulatory effect on both integration and lysogeny that is independent of binding to the attP F site. Maintenance of the lysogenic state also appears to be enhanced in the presence of Fis, as shown by the increased sensitivity of lambda prophages encoding temperature-sensitive repressors to partial thermoinduction in a fis mutant. In the presence of Xis, however, Fis binding to F interferes with integration by stimulating excision, the competing back-reaction. Since Fis stimulates both excision and integration, depending on the presence or absence of Xis, respectively, we conclude that Xis binding to X1 is the key determinant directing the formation of an excisive complex.

Caulobacter crescentus has a penicillinase which precludes the use of penicillin for mutant enrichment. However, two other antibiotics, fosfomycin and D-cycloserine, can be used to enrich for C. crescentus mutants. In enrichment procedures for C. crescentus auxotrophs, spontaneously derived mutants occur at a frequency of 5–10% among the survivors of an enrichment procedure. Consequently, large numbers of mutants are readily obtained without any need for mutagenesis. These mutants are heterogeneous both with regard to the type of mutation and to the nutritional requirement. A similar procedure has been used to isolate temperature-sensitive mutants.

Cultures of wild-type Caulobacter crescentus and strains with fla mutations representing 24 genes were pulse-labeled with 14C-amino acids and analyzed by immunoprecipitation to study the synthesis of flagellar components. Most fla mutants synthesize flagellin proteins at a reduced rate, suggesting the existence of some mechanism to prevent the accumulation of unpolymerized flagellin subunits. Two strains contain deletions that appear to remove a region necessary for this regulation. The hook protein does not seem to be subject to this type of regulation and, in addition, appears to be synthesized as a faster-sedimenting precursor. Mutations in a number of genes result in the appearance of degradation products of either the flagellin or the hook proteins. Mutations in flaA, -X, -Y, or -Z result in the production of filaments (stubs) that contain altered ratios of the flagellin proteins. In some flaA mutants, other flagellin-related proteins were assembled into the stub structures in addition to the flagellins normally present. Taken together, these analyses have begun to provide insight into the roles of individual fla genes in flagellum biogenesis in C. crescentus.

Enhancers of DNA inversion in bacteria are sites that bind specific proteins. These complexes can act at distances of more than 5 kbp to enhance the rate of site-specific recombination 200-fold. Characterization of the recombination reaction suggests that the enhancer may function to stabilize a synaptic structure that is competent for strand exchange. The role of the enhancer as an element that mediates the stabilization of specific DNA-protein configurations to provide for the action of enzymes such as recombinases is described.

Summary The alternate expression of flagellin genes in Salmonella is the result of an inversion of a 996 bp segment of chromosomal DNA. We have analyzed the components of this site-specific recombination reaction in an in vitro system derived from E. coil. Efficient Hin-mediated inversion requires the 20,000 MW Hin protein and a proteinase K-sensitive host component. The supercoiled DNA substrate must contain two 26 bp recombination sites in inverted configuration and a 60 bp sequence that increases the rate of recombination over 20-fold. This recombinational enhancer can function at many different locations and consists of at least two noncontiguous sequence domains whose relative orientation, but not precise spacing, with respect to each other is important. Synthetically derived wild-type and mutant recombination sites were constructed to analyze the sequence and structural features that are important within the recombination site.

RNA polymerase is known to undergo a cycling process during initiation of transcription in vitro in which large amounts of small oligonucleotides are released. We have used this cycling reaction to determine the 5' end of the RNA synthesized from the outer ends of the Tn5 inverted repeats. By analyzing the size of the radiolabeled oligonucleotides synthesized using different labeled nucleoside triphosphates and in reactions deficient for a particular nucleoside triphosphate, the partial 5' sequence was obtained. This sequence was correlated with the DNA sequence of the region and an unambiguous origin for the mRNA was determined. The start site for the RNA, which is located at 97 base pairs from the outer ends of the inverted repeats, was confirmed by digesting gamma-32 P-ATP labeled full length (run off) transcripts with ribonuclease T1. The resulting gamma-labeled T1 generated oligonucleotide corresponded to the predicted size determined using the cycling reaction. With the knowledge of the RNA start site, the probable translation start sites for the protein(s) known to be required for Tn5 transposition can be predicted. Possible implications of the DNA sequence with respect to the regulation of the transposase are also discussed.