Search

DNA mismatch repair (MMR) system corrects mismatched bases that are generated mainly by DNA replication errors. The repair system excises the error-containing single-stranded region and enables the re-synthesis of the strand. In the early reactions of MMR, MutL endonuclease incises the newly-synthesized/error-containing strand of the duplex to initiate the downstream excision reaction. MutL endonuclease consists of the N-terminal ATPase and C-terminal endonuclease domains. In this study, we report the crystal structure of the ATPase domain of MutL endonuclease from Aquifex aeolicus. The overall structure of the domain was similar to those of human MutL homologs and Escherichia coli MutL, although E. coli MutL has no endonuclease activity. The ATPase domain was comprised of two subdomains: the N-terminal ATP-binding subdomain and the C-terminal α-β sandwich subdomain. Site-directed mutagenesis experiment identified DNA-interacting eight basic amino acid residues, which were distributed across both the two subdomains and formed a DNA-binding cleft. Docking simulation between the structures of the ATPase and endonuclease domains generated a reliable model structure for the full-length A. aeolicus MutL, which satisfies our previous result of small-angle X-ray scattering analysis. On the basis of the model structure and further experimental results, we concluded that the two separate DNA-binding sites in the full-length A. aeolicus MutL simultaneously bind a dsDNA molecule., (Copyright © 2017 Elsevier B.V. All rights reserved.)

DNA mismatch repair (MMR) greatly contributes to genome integrity via the correction of mismatched bases that are mainly generated by replication errors. Postreplicative MMR excises a relatively long tract of error-containing single-stranded DNA. MutL is a widely conserved nicking endonuclease that directs the excision reaction to the error-containing strand of the duplex by specifically nicking the daughter strand. Because MutL apparently exhibits nonspecific nicking endonuclease activity in vitro, the regulatory mechanism of MutL has been argued. Recent studies suggest ATP-dependent conformational and functional changes of MutL, indicating that the regulatory mechanism involves the ATP binding and hydrolysis cycle. In this study, we investigated the effect of ATP binding on the structure of MutL. First, a cross-linking experiment confirmed that the N-terminal ATPase domain physically interacts with the C-terminal endonuclease domain. Next, hydrogen/deuterium exchange mass spectrometry clarified that the binding of ATP to the N-terminal domain induces local structural changes at the catalytic sites of MutL C-terminal domain. Finally, on the basis of the results of the hydrogen/deuterium exchange experiment, we successfully identified novel regions essential for the endonuclease activity of MutL. The results clearly show that ATP modulates the nicking endonuclease activity of MutL via structural rearrangements of the catalytic site. In addition, several Lynch syndrome-related mutations in human MutL homolog are located in the position corresponding to the newly identified catalytic region. Our data contribute toward understanding the relationship between mutations in MutL homolog and human disease.

Endonuclease IV (EndoIV) is an endonuclease that acts at apurinic/apyrimidinic (AP) sites and is classified as either long-type or short-type. The crystal structures of representative types of EndoIV from Geobacillus kaustophilus and Thermus thermophilus HB8 were determined using X-ray crystallography. G. kaustophilus EndoIV (the long type) had a higher affinity for double-stranded DNA containing an AP-site analogue than T. thermophilus EndoIV (the short type). Structural analysis of the two different EndoIVs suggested that a C-terminal DNA-recognition loop that is only present in the long type contributes to its high affinity for AP sites. A mutation analysis showed that Lys267 in the C-terminal DNA-recognition loop plays an important role in DNA binding.

Uracil-DNA glycosylase (UDG) removes uracil generated by the deamination of cytosine or misincorporation of deoxyuridine monophosphate. Within the UDG superfamily, a fifth UDG family lacks a polar residue in the active-site motif, which mediates the hydrolysis of the glycosidic bond by activation of a water molecule in UDG families 1-4. We have determined the crystal structure of a novel family 5 UDG from Thermus thermophilus HB8 complexed with DNA containing an abasic site. The active-site structure suggests this enzyme uses both steric force and water activation for its excision reaction. A conserved asparagine residue acts as a ligand to the catalytic water molecule. The structure also implies that another water molecule acts as a barrier during substrate recognition. Based on no significant open-closed conformational change upon binding to DNA, we propose a "slide-in" mechanism for initial damage recognition.

The cyclobutane pyrimidine dimer (CPD) is one of the major forms of DNA damage caused by irradiation with ultraviolet (UV) light. CPD photolyases recognize and repair UV-damaged DNA. The DNA recognition mechanism of the CPD photolyase has remained obscure because of a lack of structural information about DNA-CPD photolyase complexes. In order to elucidate the CPD photolyase DNA binding mode, we performed NMR analyses of the DNA-CPD photolyase complex. Based upon results from (31)P NMR measurements, in combination with site-directed mutagenesis, we have demonstrated the orientation of CPD-containing single-stranded DNA (ssDNA) on the CPD photolyase. In addition, chemical shift perturbation analyses, using stable isotope-labeled DNA, revealed that the CPD is buried in a cavity within CPD photolyase. Finally, NMR analyses of a double-stranded DNA (dsDNA)-CPD photolyase complex indicated that the CPD is flipped out of the dsDNA by the enzyme, to gain access to the active site.

DNA mismatch repair is an excision system that removes mismatched bases chiefly generated by replication errors. In this system, MutL endonucleases direct the excision reaction to the error-containing strand of the duplex by specifically incising the newly synthesized strand. Both bacterial homodimeric and eukaryotic heterodimeric MutL proteins belong to the GHKL ATPase/kinase superfamily that comprises the N-terminal ATPase and C-terminal dimerization regions. Generally, the GHKL proteins show large ATPase cycle-dependent conformational changes, including dimerization-coupled ATP binding of the N-terminal domain. Interestingly, the ATPase domain of human PMS2, a subunit of the MutL heterodimer, binds ATP without dimerization. The monomeric ATP-bound state of the domain has been thought to be characteristic of heterodimeric GHKL proteins. In this study, we characterized the ATP-bound state of the ATPase domain from the Aquifex aeolicus MutL endonuclease, which is a homodimeric GHKL protein unlike the eukaryotic MutL. Gel filtration, dynamic light scattering, and small-angle X-ray scattering analyses clearly showed that the domain binds ATP in a monomeric form despite its homodimeric nature. This indicates that the uncoupling of dimerization and ATP binding is a common feature among bacterial and eukaryotic MutL endonucleases, which we suggest is closely related to the molecular mechanisms underlying mismatch repair. [ABSTRACT FROM AUTHOR]

In the initial steps of DNA mismatch repair, MutS recognizes a mismatched base and recruits the latent endonuclease MutL onto the mismatch-containing DNA in concert with other proteins. MutL then cleaves the error-containing strand to introduce an entry point for the downstream excision reaction. Because MutL has no intrinsic ability to recognize a mismatch and discriminate between newly synthesized and template strands, the endonuclease activity of MutL is strictly regulated by ATP-binding in order to avoid nonspecific degradation of the genomic DNA. However, the activation mechanism for its endonuclease activity remains unclear. In this study, we found that the coexistence of a mismatch, ATP and MutS unlocks the ATP-binding-dependent suppression of MutL endonuclease activity. Interestingly, ATPase-deficient mutants of MutS were unable to activate MutL. Furthermore, wild-type MutS activated ATPase-deficient mutants of MutL less efficiently than wild-type MutL. We concluded that ATP hydrolysis by MutS and MutL is involved in the mismatch-dependent activation of MutL endonuclease activity. [ABSTRACT FROM AUTHOR]

Alkylation is a type of stress that is fatal to cells. However, cells have various responses to alkylation. Alkyltransferase-like (ATL) protein is a novel protein involved in the repair of alkylated DNA; however, its repair mechanism at the molecular level is unclear. DNA microarray analysis revealed that the upregulation of 71 genes because of treatment with an alkylating agent N-methyl-N′-nitro-N-nitrosoguanidine was related to the presence of TTHA1564, the ATL protein from Thermus thermophilus HB8. Affinity chromatography showed a direct interaction of purified TTHA1564 with purified RNA polymerase holoenzyme. The amino acid sequence of TTHA1564 is homologous to that of the C-terminal domain of Ada protein, which acts as a transcriptional activator. These results suggest that TTHA1564 might act as a transcriptional regulator. The results of DNA microarray analysis also implied that the alkylating agent induced oxidation stress in addition to alkylation stress. [ABSTRACT FROM PUBLISHER]

Deoxyribonucleoside triphosphate triphosphohydrolase from Thermus thermophilus (Tt-dNTPase) has a unique regulatory mechanism for the degradation of deoxyribonucleoside triphosphates (dNTPs). Whereas the Escherichia coli homologue specifically hydrolyzes dGTP alone, dNTPs act as both substrate and activator for Tt-­dNTPase. Here, the crystal structure of Tt-dNTPase has been determined at 2.2 Å resolution, representing the first report of the tertiary structure of a dNTPase homologue belonging to the HD superfamily, a diverse group of metal-dependent phosphohydrolases that includes a variety of uncharacterized proteins. This enzyme forms a homohexamer as a double ring of trimers. The subunit is composed of 19 α-­helices; the inner six helices include the region annotated as the catalytic domain of the HD superfamily. Structural comparison with other HD-superfamily proteins indicates that a pocket at the centre of the inner six helices, formed from highly conserved charged residues clustered around a bound magnesium ion, constitutes the catalytic site. Tt-dNTPase also hydrolyzed noncanonical dNTPs, but hardly hydrolyzed dNDP and dNMP. The broad substrate specificity for different dNTPs might be rationalized by the involvement of a flexible loop during molecular recognition of the base moiety. Recognition of the triphosphate moiety crucial for the activity might be attained by highly conserved positively charged residues. The possible mode of dNTP binding is discussed in light of the structure. [ABSTRACT FROM AUTHOR]

Presents a study which characterized thermostable RecA protein from Thermus thermopilus. Analysis of the protein's interaction with single-stranded DNA; Overexpression of Thermus thermophilus recA gene in Escherichia Coli.

The single-stranded DNA-dependent ATPase activity of Escherichia coli RecA protein, especially its cooperativity for ATP, was investigated. To measure the ATPase activity in detail, the methods and reaction conditions for the ATPase assay were reexamined. Under conditions where RecA protein always showed a maximal rate of ATP hydrolysis, its poly(dT)-dependent ATPase activity was measured. At 25°C, increasing the concentration of RecA protein from 0.3 to 1.0 μM increased the turnover number (kcat) from 0.16 to 0.19 s−1 and the Hill coefficient (nH) for ATP from 9.3 to 11.6. At 0.5 μM RecA protein, increasing the temperature from 25 to 37°C increased kcat from 0.18 to 0.35 s−1 but decreased nH from 9.8 to 6.6. Interestingly, the ATPase activity of RecA protein measured in this study showed much higher cooperativity for ATP than those reported to date. Furthermore, the nH value of 11.6 for ATP obtained here was the highest of any ATPase reported so far. These results suggest that the binding of an ATP molecule to a RecA molecule within a nucleoprotein helical filament causes structural change of many other neighboring RecA molecules. This implies that ATP binding induces structural change of the whole nucleoprotein helical filament. Finally, we demonstrated that analysis of cooperativity is useful for revealing how a protein composed of many subunits functions as a whole. [ABSTRACT FROM AUTHOR]

RecA2278-5 is a mutant RecA protein (RecAmut) bearing two amino acid substitutions, Gly-278 to Thr and Val-275 to Phe, in the α-helix H of the C-terminal sub-domain of the protein, recA2278-5 mutant cells are unusual in that they are thermosensitive for recombination but almost normal for DNA repair of UV damage and the SOS response. Biochemical analysis of purified RecAmut protein revealed that its temperature sensitivity is suppressed by prior binding of this protein to its ligand. In fact, the preheating of RecAmut protein for several minutes at a restrictive temperature (42°C) in the absence of ATP resulted in inhibition at 42°C of many activities related to homologous recombination including ss- and dsDNA binding, high-affinity binding for ATP, ss- or dsDNA-dependent ATPase, RecA-RecA interaction, and strand transfer capability. The binary complex RecAmut::ATP under the same conditions showed a decrease in only two activities, i.e. dsDNA binding and high-affinity binding for ATP. Besides ATP, sodium acetate (1.5 M) was shown to be another factor that can stabilize the RecAmut protein at 42°C, judging by restoration of its DNA-free ATPase activity. The similarity of influence of high salt (with its non-specific binding) and ATP (binding specifically) on the apparent protein folding stability suggests that the structural stability of the RecA C-terminal domain is one of the conditions for correct interaction between RecA protein and ATP in the RecA::ATP::ssDNA presynaptic complex formation. The decrease in affinity for ATP was suggested to be the factor that determined a particular recombinational (but not repair) thermosensitivity of the RecAmut protein. Finally, we show that the stability of C-terminal domain appeared to be necessary for the dsDNA-binding activity of the protein. [ABSTRACT FROM AUTHOR]

Cyclobutane pyrimidine dimer (CPD) photolyases use light to repair CPDs. For efficient light absorption, CPD photolyases use a second chromophore. We purified Thermus thermophilus CPD photolyase with its second chromophore. UV-visible absorption spectra, reverse-phase HPLC, and NMR analyses of the chromophores revealed that the second chromophore of the enzyme is flavin mononucleotide (FMN). To clarify the role of FMN in the CPD repair reaction, the enzyme without FMN (Enz-FMN(-) and that with a stoichiometric amount of FMN (Enz-FMN(+)) were both successfully obtained. The CP D repair activity of Enz-FMN(+) was higher than that of Enz-FMN(-), and the CPD repair activity ratio of Enz-FMN(+ ) and Enz-FMN(-) was dependent on the wavelength of light. These results suggest that FMN increases the light absorption efficiency of the enzyme. NMR analyses of Enz-FMN(+) and Enz-FMN(-) revealed that the binding mode of FMN is similar to that of 7,8-didemethyl-8-hydroxy-5-deazariboflavin in Anacystis nidulans CPD photolyase, and thus a direct electron transfer between FMN and CPD is not likely to occur. Based on these results, we concluded that FMN acts as a highly efficient light harvester that gathers light and transfers the energy to FAD. [ABSTRACT FROM AUTHOR]

DNA recombination events need to be strictly regulated, because an increase in the recombinational frequency causes unfavorable alteration of genetic information. Recent studies revealed the existence of a novel anti-recombination enzyme, MutS2. However, the mechanism by which MutS2 inhibits homologous recombination has been unknown. Previously, we found that Thermus thermophilus MutS2 (ttMutS2) harbors an endonuclease activity and that this activity is confined to the C-terminal domain, whose amino acid sequence is widely conserved in a variety of proteins with unknown function from almost all organisms ranging from bacteria to man. In this study, we determined the crystal structure of the ttMutS2 endonuclease domain at 1.7-Å resolution, which resembles the structure of the DNase I-like catalytic domain of Escherichia coli RNase E, a sequence-nonspecific endonuclease. The N-terminal domain of ttMutS2, however, recognized branched DNA structures, including the Holliday junction and D-loop structure, a primary intermediate in homologous recombination. The full-length of ttMutS2 digested the branched DNA structures at the junction. These results indicate that ttMutS2 suppresses homologous recombination through a novel mechanism involving resolution of early intermediates. [ABSTRACT FROM AUTHOR]

DNA mismatch repair corrects mismatched base pairs mainly caused by replication error. Recent studies revealed that human MutL endonuclease, hPMS2, plays an essential role in the repair. However, there has been little biochemical analysis of the MutL endonuclease. In particular, it is unknown for what the MutL utilizes ATP binding and hydrolyzing activity. Here we report the detailed functional analysis of Thermus thermophilus MutL (ttMutL). ttMutL exhibited an endonuclease activity that decreased on alteration of Asp-364 in ttMutL to Asn. The biochemical characteristics of ttMutL were significantly affected on ATP binding, which suppressed nonspecific DNA digestion and promoted the mismatch- and MutS-dependent DNA binding. The inactivation of the cysteinyl residues in the C-terminal domain resulted in the perturbation in ATP-dependent regulation of the endonuclease activity, although the ATP-binding motif is located in the N-terminal domain. Complementation experiments revealed that the endonuclease activity of ttMutL and its regulation by ATP binding are necessary for DNA repair in vivo. [ABSTRACT FROM AUTHOR]

Abstract: Oxidatively damaged DNA precursors (deoxyribonucleotides) are formed by reactive oxygen species. After the damaged DNA precursors are incorporated into DNA, they might be removed by DNA repair enzymes. In this study, to examine whether a nucleotide excision repair enzyme, Escherichia coli UvrABC, could suppress the mutations induced by oxidized deoxyribonucleotides in vivo, oxidized DNA precursors, 8-hydroxy-2′-deoxyguanosine 5′-triphosphate and 2-hydroxy-2′-deoxyadenosine 5′-triphosphate, were introduced into uvrA, uvrB, and uvrC E. coli strains, and mutations in the chromosomal rpoB gene were analyzed. Unexpectedly, these oxidized DNA precursors induced mutations only slightly in the uvrA and uvrB strains. In contrast, effect of the uvrC-deficiency was not observed. Next, mutT, mutT/uvrA, and mutT/uvrB E. coli strains were treated with H2O2, and the rpoB mutant frequencies were calculated. The frequency of the H2O2-induced mutations was increased in all of the strains tested; however, the increase was three- to four-fold lower in the mutT/uvrA and mutT/uvrB strains than in the mutT strain. Thus, UvrA and UvrB are involved in the enhancement, but not in the suppression, of the mutations induced by these oxidized deoxyribonucleotides. These results suggest a novel role for UvrA and UvrB in the processing of oxidative damage. [Copyright &y& Elsevier]

Cyclobutane pyrimidine dimer (CPD) photolyases, which contain FAD as a cofactor, use light to repair CPDs. We performed structural analyses of the catalytic site of the Thermus thermophilus CPD photolyase-DNA complex, using FAD-induced paramagnetic relaxation enhancement (PRE). The distances between the tryptophan residues and the FAD calculated from the PRE agree well with those observed in the x-ray structure (with an error of <3 Å). Subsequently, a single-stranded DNA containing 13C-labeled CPD was prepared, and the FAD-induced PRE of the NMR resonances from the CPD lesion in complex with the CPD photolyase was investigated. The distance between the FAD and the CPD calculated from the PRE is 16 ± 3 Å. The FAD-induced PRE was also observed in the CPD photolyase-double. stranded DNA complex. Based on these results, a model of the CPD photolyase. DNA complex was constructed, and the roles of Arg-201, Lys-240, Trp-247, and Trp-353 in the CPD-repair reaction are discussed. [ABSTRACT FROM AUTHOR]

Uracil-DNA glycosylase (UDG; EC 3.2.2.-) removes uracil from DNA to initiate DNA base excision repair. Since hydrolytic deamination of cytosine to uracil is one of the most frequent DNA-damaging events in all cells, UDG is an essential enzyme for maintaining the integrity of genomic information. For the first time, we report the crystal structure of a family 4 UDG from Thermus thermophilus HB8 (TthUDG) complexed with uracil, solved at 1.5 A˚ resolution. As opposed to UDG enzymes in its other families, TthUDG possesses a [4Fe–4S] cluster. This iron–sulfur cluster, which is distant from the active site, interacts with loop structures and has been suggested to be unessential to the activity but necessary for stabilizing the loop structures. In addition to the iron–sulfur cluster, salt-bridges and ion pairs on the molecular surface and the presence of proline on loops and turns is thought to contribute to the enzyme's thermostability. Despite very low levels of sequence identity with Escherichia coli and human UDGs (family 1) and E. coli G:T/U mismatch-specific DNA glycosylase (MUG) (family 2), the topology and order of secondary structures of TthUDG are similar to those of these distant relatives. Furthermore, the coordinates of the core structure formed by β-strands are almost the same. Positive charge is distributed over the active-site groove, where TthUDG would bind DNA strands, as do UDG enzymes in other families. TthUDG recognizes uracil specifically in the same manner as does human UDG (family 1), rather than guanine in the complementary strand DNA, as does E. coli MUG (family 2). These results suggest that the mechanism by which family 4 UDGs remove uracils from DNA is similar to that of family 1 enzymes. [Copyright &y& Elsevier]

The extremely thermophilic bacterium Thermus thermophilus HB8, which belongs to the phylum Deinococcus-Thermus, has an open reading frame encoding a protein belonging to the cyclic AMP (cAMP) receptor protein (CRP) family present in many bacteria. The protein named T. thermophilus CRP is highly homologous to the CRP family proteins from the phyla Firmicutes, Actinobacteria, and Cyanobacteria, and it forms a homodimer and interacts with cAMP. CRP mRNA and intracellular cAMP were detected in this strain, which did not drastically fluctuate during cultivation in a rich medium. The expression of several genes was altered upon disruption of the T. thermophilus CRP gene. We found six CRP-cAMP-dependent promoters in in vitro transcription assays involving DNA fragments containing the upstream regions of the genes exhibiting decreased expression in the CRP disruptant, indicating that the CRP is a transcriptional activator. The consensus T. thermophilus CRP-binding site predicted upon nucleotide sequence alignment is 5′-(C/T)NNG(G/T)(G/T)C(A/C)N(A/T)NNTCACAN(G/C)(G/C)-3′. This sequence is unique compared with the known consensus binding sequences of CRP family proteins. A putative -10 hexamer sequence resides at 18 to 19 bp downstream of the predicted T. thermophilus CRP-binding site. The CRP-regulated genes found in this study comprise clustered regularly interspaced short palindromic repeat (CRISPR)-associated (cas) ones, and the genes of a putative transcriptional regulator, a protein containing the exonuclease III-like domain of DNA polymerase, a GCN5-related acetyltransferase homolog, and T. thermophilus-specific proteins of unknown function. These results suggest a role for cAMP signal transduction in T. thermophilus and imply the T. thermophilus CRP is a cAMP-responsive regulator. [ABSTRACT FROM AUTHOR]