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It has been shown in recent years that many repeated sequences in the genome are expressed as RNA transcripts, although the role of such RNAs is poorly understood. Some isolated and tandem repeats (satellites) have been found to be transcribed, such as mammalian Alu sequences and telomeric/centromeric satellites in different species. However, there is no detailed study on the eventual transcription of the interspersed satellites found in many species. Therefore, we decided to study for the first time the transcription of the abundant DNA satellites in the bacterium Bacillus coagulans and in the nematode Caenorhabditis elegans. We have updated the data for C. elegans satellites using the latest version of the genome. We analyzed the transcription of satellites in both species in available RNA-seq results and found that they are widely transcribed. Our demonstration that satellite RNAs are transcribed adds a new family of non-coding RNAs. This is a field that requires further investigation and will provide a deeper understanding of gene expression and control. This work was supported by Ministerio de Ciencia e Innovación, Spain [Project RTI2018-094403-B-C33 funded by MCIN/ AEI 10.13039/501100011033/ FEDER].

El presente artículo es una investigación de carácter descriptivo que tiene como objetivo principal analizar el perfil y la intención emprendedora de estudiantes del último año de todas las carreras ofertadas por la Universidad Nur en Santa Cruz-Bolivia, con un análisis crítico y reflexivo sobre el tema, buscando identificar cuáles son los factores considerados como promotores de la conducta emprendedora y la influencia que ejerce el entorno en la orientación hacia la intención emprendedora. Se ha aplicado un cuestionario estructurado a una muestra de 220 estudiantes. Entre los resultados se destaca que cursos de emprendedurismo tiene un impacto sobre la Autoeficacia Emprendedora (AF) y la Intención Emprendedora (IE), sin relación directa con las variables Norma Subjetiva (NS) y Actitud Emprendedora (AE). Finalmente se presenta la discusión y conclusiones relacionados a las variables. PALABRAS CLAVES: Emprendedor, Educación Emprendedora, Intención Emprendedora.

Little is known about DNA tandem repeats across prokaryotes. We have recently described an enigmatic group of tandem repeats in bacterial genomes with a constant repeat size but variable sequence. These findings strongly suggest that tandem repeat size in some bacteria is under strong selective constraints. Here, we extend these studies and describe tandem repeats in a large set of Bacillus. Some species have very few repeats, while other species have a large number. Most tandem repeats have repeats with a constant size (either 52 or 20–21 nt), but a variable sequence. We characterize in detail these intriguing tandem repeats. Individual species have several families of tandem repeats with the same repeat length and different sequence. This result is in strong contrast with eukaryotes, where tandem repeats of many sizes are found in any species. We discuss the possibility that they are transcribed as small RNA molecules. They may also be involved in the stabilization of the nucleoid through interaction with proteins. We also show that the distribution of tandem repeats in different species has a taxonomic significance. The data we present for all tandem repeats and their families in these bacterial species will be useful for further genomic studies.

We present a new structure of a DNA dodecamer obtained in the presence of Ni2+ ions. The DNA forms Ni-guanine cross-links between neighboring molecules. Our results show that an adequate dosage of Ni2+ may help to form well-defined DNA nanostructures. We also compare our structure with other dodecamers which present unique features and also crystallize in trigonal unit cells, strongly influenced by the counterions associated with DNA. In all cases, the DNA duplexes form parallel pseudo-helical columns in the crystal, similar to DNA-protamine and native DNA fibers.

DNA tandem repeats, or satellites, are well described in eukaryotic species, but little is known about their prevalence across prokaryotes. Here, we performed the most complete characterization to date of satellites in bacteria. We identified 121,638 satellites from 12,233 fully sequenced and assembled bacterial genomes with a very uneven distribution. We also determined the families of satellites which have a related sequence. There are 85 genomes that are particularly satellite rich and contain several families of satellites of yet unknown function. Interestingly, we only found two main types of noncoding satellites, depending on their repeat sizes, 22/44 or 52¿nucleotides (nt). An intriguing feature is the constant size of the repeats in the genomes of different species, whereas their sequences show no conservation. Individual species also have several families of satellites with the same repeat length and different sequences. This result is in marked contrast with previous findings in eukaryotes, where noncoding satellites of many sizes are found in any species investigated. We describe in greater detail these noncoding satellites in the spirochete Leptospira interrogans and in several bacilli. These satellites undoubtedly play a specific role in the species which have acquired them. We discuss the possibility that they represent binding sites for transcription factors not previously described or that they are involved in the stabilization of the nucleoid through interaction with proteins. IMPORTANCE We found an enigmatic group of noncoding satellites in 85 bacterial genomes with a constant repeat size but variable sequence. This pattern of DNA organization is unique and had not been previously described in bacteria. These findings strongly suggest that satellite size in some bacteria is under strong selective constraints and thus that satellites are very likely to play a fundamental role. We also provide a list and properties of all satellites in 12,233 genomes, which may be used for further genomic analysis. This work was supported by Ministerio de Ciencia e Innovación–Agencia Estatal de Investigación, Spain (projects TIN2015-69175-C4-3-R and RTI2018-094403-B-C33), and FEDER.

Little is known about DNA tandem repeats across prokaryotes. We have recently described an enigmatic group of tandem repeats in bacterial genomes with a constant repeat size but variable sequence. These findings strongly suggest that tandem repeat size in some bacteria is under strong selective constraints. Here, we extend these studies and describe tandem repeats in a large set of Bacillus. Some species have very few repeats, while other species have a large number. Most tandem repeats have repeats with a constant size (either 52 or 20–21 nt), but a variable sequence. We characterize in detail these intriguing tandem repeats. Individual species have several families of tandem repeats with the same repeat length and different sequence. This result is in strong contrast with eukaryotes, where tandem repeats of many sizes are found in any species. We discuss the possibility that they are transcribed as small RNA molecules. They may also be involved in the stabilization of the nucleoid through interaction with proteins. We also show that the distribution of tandem repeats in different species has a taxonomic significance. The data we present for all tandem repeats and their families in these bacterial species will be useful for further genomic studies. This work was supported by Ministerio de Ciencia e Innovación –Agencia Estatal de Investigación, Spain (Project RTI2018-094403-B-C33) and FEDER.

Repetitive genome regions have been difficult to sequence, mainly because of the comparatively small size of the fragments used in assembly. Satellites or tandem repeats are very abundant in nematodes and offer an excellent playground to evaluate different assembly methods. Here, we compare the structure of satellites found in three different assemblies of the Caenorhabditis elegans genome: the original sequence obtained by Sanger sequencing, an assembly based on PacBio technology, and an assembly using Nanopore sequencing reads. In general, satellites were found in equivalent genomic regions, but the new long-read methods (PacBio and Nanopore) tended to result in longer assembled satellites. Important differences exist between the assemblies resulting from the two long-read technologies, such as the sizes of long satellites. Our results also suggest that the lengths of some annotated genes with internal repeats which were assembled using Sanger sequencing are likely to be incorrect.

Repetitive genome regions have been difficult to sequence, mainly because of the comparatively small size of the fragments used in assembly. Satellites or tandem repeats are very abundant in nematodes and offer an excellent playground to evaluate different assembly methods. Here, we compare the structure of satellites found in three different assemblies of the Caenorhabditis elegans genome: the original sequence obtained by Sanger sequencing, an assembly based on PacBio technology, and an assembly using Nanopore sequencing reads. In general, satellites were found in equivalent genomic regions, but the new long-read methods (PacBio and Nanopore) tended to result in longer assembled satellites. Important differences exist between the assemblies resulting from the two long-read technologies, such as the sizes of long satellites. Our results also suggest that the lengths of some annotated genes with internal repeats which were assembled using Sanger sequencing are likely to be incorrect.

The availability of the genome sequence of the unisexual (male-female) Caenorhabditis nigoni offers an opportunity to compare its non-coding features with the related hermaphroditic species Caenorhabditis briggsae; to understand the evolutionary dynamics of their tandem repeat sequences (satellites), as a result of evolution from the unisexual ancestor. We take advantage of the previously developed SATFIND program to build satellite families defined by a consensus sequence. The relative number of satellites (satellites/Mb) in C. nigoni is 24.6% larger than in C. briggsae. Some satellites in C. nigoni have developed from a proto-repeat present in the ancestor species and are conserved as an isolated sequence in C. briggsae. We also identify unique satellites which occur only once and joint satellite families with a related sequence in both species. Some of these families are only found in C. nigoni, which indicates a recent appearance; they contain conserved adjacent 5' and 3' regions, which may favor transposition. Our results show that the number, length and turnover of satellites are restricted in the hermaphrodite C. briggsae when compared with the unisexual C. nigoni. We hypothesize that this results from differences in unequal recombination during meiotic chromosome pairing, which limits satellite turnover in hermaphrodites. We are most thankful to Erich M. Schwarz for providing the C. nigoni genome data and a preprint of their work ahead of publication. We also thank M. Mar Albà and Ben Lehner for fruitful discussions and advice. This work was supported in part by project TIN2015-69175-C4-3-R (MINECO-FEDER, UE), including funds for covering the costs to publish in open access.

The crystal structure of the telomeric sequence d(UBrAGG) interacting with an anthraquinone derivative has been solved by MAD. In all previously studied complexes of intercalating drugs, the drug is usually sandwiched between two DNA base pairs. Instead, the present structure looks like a crystal of stacked anthraquinone molecules in which isolated base pairs are intercalated. Unusual base pairs are present in the structure, such as G·G and A·UBr reverse Watson-Crick base pairs. © 2010 International Union of Crystallography All rights reserved., We acknowledge support from the European Project MPRN-CT-2000-00009 and from the Ministerio de Ciencia e Innovación, project No. BFU 2009-10380.

The coiled-coil structure formed by the complex of the DNA duplex d(ATATATATAT)2with pentamidine is presented. The duplex was found to have a mixed structure containing Watson–Crick and Hoogsteen base pairs. The drug stabilizes the coiled coil through the formation of cross-links between neighbouring duplexes. The central part of the drug is found in the minor groove as expected, whereas the charged terminal amidine groups protrude and interact with phosphates from neighbouring molecules. The formation of cross-links may be related to the biological effects of pentamidine, which is used as an antiprotozoal agent in trypanosomiasis, leishmaniasis and pneumonias associated with AIDS. The DNA sequence that was used is highly abundant in most eukaryotic genomes. However, very few data are available on DNA sequences which only contain A·T base pairs.

The purpose of this work is to determine the most frequent short sequences in non-coding DNA. They may play a role in maintaining the structure and function of eukaryotic chromosomes. We present a simple method for the detection and analysis of such sequences in several genomes, including Arabidopsis thaliana, Caenorhabditis elegans, Drosophila melanogaster and Homo sapiens. We also study two chromosomes of man and mouse with a length similar to the whole genomes of the other species. We provide a list of the most common sequences of 9-14 bases in each genome. As expected, they are present in human Alu sequences. Our programs may also give a graph and a list of their position in the genome. Detection of clusters is also possible. In most cases, these sequences contain few alternating regions. Their intrinsic structure and their influence on nucleosome formation are not known. In particular, we have found new features of short sequences in C. elegans, which are distributed in heterogeneous clusters. They appear as punctuation marks in the chromosomes. Such clusters are not found in either A. thaliana or D. melanogaster. We discuss the possibility that they play a role in centromere function and homolog recognition in meiosis.

We present the structure of the duplex formed by a fragment of auto-complementary DNA with the sequence d(CGTTAATTAACG). The structure was determined by X-ray crystallography. Up to date it is the first structure presenting the interaction between a DNA oligonucleotide and manganese ions. The presence of Mn 2+ creates bonds among the N7 atom of guanines and phosphates. These bonds stabilize and determine the crystallographic network in a P3 2 space group, unusual in oligonucleotide crystals. The crystal structure observed is compared with those found in the presence of Mg 2+ , Ca 2+ and Ni 2+ , which show different kinds of interactions. The double helices show end-to-end interactions, in a manner that the terminal guanines interact with the minor groove of the neighboring duplex, while the terminal cytosines are disordered. We have chosen this sequence since it contains a TTAA repeat. Such repeats are very rare in all genomes. We suggest that this sequence may be very susceptible to the formation of closely spaced thymine dimers.

The crystal structure of N-α-acetyl-l-arginine ethylamide perchlorate (molecular formula: C10H21N5O2 ± HClO4) has been determined from X-ray diffraction data. This arginine derivative has the two charged ends substituted by peptide units and thus becomes representative of an arginyl residue in a protein. The crystals are orthorhombic, space group P21,21,21, a = 7.485 (2), b = 28.623 (5), c = 7.261 (3) A, Z = 4. The structure was solved by direct methods and refined by full matrix least-squares using 559 reflections to an R = 0.052. The molecule shows two planar peptide units with φ and o values in the β pleated sheet region. The side chain torsion angles are in trans conformation, except the χ1, angle which is in the gauche(-) region. This observation further illustrates that this is one of the favoured side chain conformations of arginine, together with the all-trans conformation. Symmetric double hydrogen bonds are observed between the guanidinium group and perchlorate anions. The H(N) and oxygen atoms of the amide groups also form hydrogen bonds.

Thin plates of L-lysyl-L-valine hydrochloride (C11H24N3O3Cl) were obtained using the vapour diffusion technique and analysed by X-ray diffraction. The unit cell is orthorhombic, space group P212121, a = 5.465(6)A, b = 19.657(4) A, c = 13.522(2) A, V = 1452.6(2.1) A3 and Z = 4. The structure was solved by direct methods and refined to an agreement factor of 6.7% for 939 reflections with I > 3 σ(I). The lysine side chain conformation (g- g- tt) has never been found in peptide crystal structures, although it has been reported to occur in proteins. A network of hydrogen bonds between peptide molecules spreads along the a and c directions while no direct bonds are observed to occur between peptides along the b axis direction. This asymmetric pattern of interactions correlates with the crystal morphology.

The crystal structure of N-α-acetyl-L-arginine-methyl ester hydrochloride has been determined from X-ray diffraction data. The crystals are orthorhombic, space group P212121, a = 15.746 (4), b = 11.569 (4), c = 7.158 (3) A, Z = 4. The structure was solved by direct methods (MULTAN) and refined by fullmatrix least-squares to an R = 0.070 for all observed reflections. The molecule shows a planar peptidic unit, with a distortion of H(N1) due to hydrogen bond interaction. The side chain is in the low energy all-trans conformation. The guanidinium group forms three hydrogen bonds with the carbonyl oxygens of two neighbouring molecules. A direct interaction of the peptidic NH with the chloride ions is observed. The latter ions also interact with the guanidinium groups, neutralizing their charges.

We present the crystal structure of the DNA duplex formed by d(ATATATCT). The crystals contain seven stacked antiparallel duplexes in the asymmetric unit with A.T Hoogsteen base pairs. The terminal CT sequences bend over so that the thymines enter the minor groove and form a hydrogen bond with thymine 2 of the complementary strand in the Hoogsteen duplex. Cytosines occupy extra-helical positions; they contribute to the crystal lattice through various kinds of interactions, including a unique CAA triplet. The presence of thymine in the minor groove apparently contributes to the stability of the DNA duplex in the Hoogsteen conformation. These observations open the way toward finding under what conditions the Hoogsteen duplex may be stabilized in vivo. The present crystal structure also confirms the tendency of A.T-rich oligonucleotides to crystallize as long helical stacks of duplexes.

A model is developed to calculate the influence of the shear rate on the intrinsic viscosity of dilute solutions of nondraining polymer coils. The polymer molecule is substituted by an equivalent ellipsoid with internal flow currents such that the macromolecule yields to the liquid flow and follows it up to a certain extent. The dimensions of the ellipsoid are obtained by equating the hydrodynamic deforming forces to the statistical restoring forces of the polymer chain. The intrinsic viscosity of the solution can then be calculated as a function of the shear rate.

Deoxyribonucleohistone from sea urchin sperm has a marked tendency to form a gel upon suspension in distilled water. The gel can be disrupted by shear and soluble nucleoprotein is thus obtained. Soluble proteins from the microtubules of the sperm tail have a tendency to contaminate this preparation. At the same time they show some affinity with the nucleohistone. Histones are obtained from treated sperm and differences are reproducibly found at various stages of the preparation. From these results the factors which determine gel formation are discussed. The melting curves of the nucleoprotein are also studied. At low ionic strength a bimodal curve is obtained, probably due to an irregular distribution of histones on the DNA molecule. At high ionic strength a bimodal curve is also found, but in this case it is due to the turbidity of the nucleprotein solution.

Background The high density of tandem repeat sequences (satellites) in nematode genomes and the availability of genome sequences from several species in the group offer a unique opportunity to better understand the evolutionary dynamics and the functional role of these sequences. We take advantage of the previously developed SATFIND program to study the satellites in four Caenorhabditis species and investigate these questions. Methods The identification and comparison of satellites is carried out in three steps. First we find all the satellites present in each species with the SATFIND program. Each satellite is defined by its length, number of repeats, and repeat sequence. Only satellites with at least ten repeats are considered. In the second step we build satellite families with a newly developed alignment program. Satellite families are defined by a consensus sequence and the number of satellites in the family. Finally we compare the consensus sequence of satellite families in different species. Results We give a catalog of individual satellites in each species. We have also identified satellite families with a related sequence and compare them in different species. We analyze the turnover of satellites: they increased in size through duplications of fragments of 100-300 bases. It appears that in many cases they have undergone an explosive expansion. In C. elegans we have identified a subset of large satellites that have strong affinity for the centromere protein CENP-A. We have also compared our results with those obtained from other species, including one nematode and three mammals. Conclusions Most satellite families found in Caenorhabditis are species-specific; in particular those with long repeats. A subset of these satellites may facilitate the formation of kinetochores in mitosis. Other satellite families in C. elegans are either related to Helitron transposons or to meiotic pairing centers.

Protamine-like proteins constitute a group of sperm nuclear basic proteins that have been shown to be related to somatic linker histones (histone H1 family). Like protamines, they usually replace the chromatin somatic histone complement during spermiogenesis; hence their name. Several of these proteins have been characterized to date in invertebrate organisms, but information about their occurrence and characterization in vertebrates is still lacking. In this sense, the genus Mullus is unique, as it is the only known vertebrate that has its sperm chromatin organized by virtually only protamine-like proteins. We show that the sperm chromatin of this organism is organized by two type I protamine-like proteins (PL-I), and we characterize the major protamine-like component of the fish Mullus surmuletus (striped red mullet). The native chromatin structure resulting from the association of these proteins with DNA was studied by micrococcal nuclease digestion as well as electron microscopy and X-ray diffraction. It is shown that the PL-I proteins organize chromatin in parallel DNA bundles of different thickness in a quite distinct arrangement that is reminiscent of the chromatin organization of those organisms that contain protamines (but not histones) in their sperm.

The structure of the oligonucleotide duplex d(AAATATTT)(2) has been obtained in two crystal forms. In both cases the duplexes show Watson-Crick base pairs and are organized as a helical arrangement of stacked oligonucleotides. The helices contain either 11 duplexes in seven turns (form I) or 14 duplexes in nine turns (form II). As a result, the crystals have rather large unit cells. Such an organization of DNA duplexes has not been previously described in oligonucleotide crystal structures. The columns of stacked duplexes are organized in a pseudohexagonal arrangement. They do not show any direct lateral interactions; instead, these are indirectly mediated by shared and disordered hydrated ions. Such interactions are similar to those found in DNA fibres. The structural parameters of the individual base steps coincide with those found in other structures which contain CG terminal base pairs.

The crystal structures of the hexamer duplex d(CGTACG)(2) complexed with the intercalating anthraquinone derivative 1,5-bis[3-(diethylamino)propionamido]anthracene-9,10-dione and the acridine derivative 9-acridinyl tetralysine have been solved at 2.0- and 1.4-A resolution, respectively. In both cases, the drugs adopt multiple orientations within a large DNA cavity constituted by two groups of four approximately coplanar bases. Cations play a pivotal role in the crystal structure. Both complexes crystallise in the presence of Co(2+), Ba(2+) and Na(+) ions. They reveal at least two different types of coordination environments: (1) specific sites for Co(2+) interacting with N7 of guanine; (2) a central ionic site formed by four phosphate groups, which can be occupied by different ions. One more ionic site that is not always occupied by ions is also visible in the electron density map. All of them play a role in the crystal structure.

Xenopus laevis nucleoplasmin is a pentameric nuclear chaperone. The relation between the structure and the multifunctional aspects of the molecule has not yet been clearly established. In the course of analysing a C-terminally His-tagged recombinant version of the region equivalent to the trypsin resistant core (r-NP142) of the molecule, we found that this domain exhibited a substantially decreased oligomerization potential. To better understand the role of the three cysteines of nucleoplasmin on its pentameric functional structure, we have selectively mutated these residues to serine and generated three mutants (C15S, C35S, and C45S) both for the complete recombinant nucleoplasmin (r-NP) and the truncated r-NP142 non-tagged forms. We demonstrate that there are no disulphide bridges stabilizing either the monomer or the pentamer. Neither C15S nor C35S has any structural effects, while the mutation C45S abolishes the ability of r-NP142 to pentamerize. This structural impairment suggests that hydrophobic interactions of Cys 45 are critical for the stability of the protein. Our studies allow to analyse for the first time the structural and functional properties of nucleoplasmin in its monomeric form.

We report the coiled-coil structure of DNA, which is generated by the dodecanucleotide d(ATATATATATAT). The structure has been determined by single-crystal x-ray crystallography. The molecules form duplexes with single-stranded overhangs, which associate with neighbor molecules and give rise to infinite double helices in a coiled-coil conformation, with staggered nicks in both strands. The coiled coils have six dodecamer duplexes per turn. Despite the presence of nicks, the structure is very rigid. Statistical disorder is present, which gives rise to continuous scattering along the layer lines. This observation can be interpreted by the classical theory of coiled coils developed for proteins, which we apply here to a DNA structure. A clear splitting of the original layer lines of the DNA double helix is detected. The structure we have found adds a previously unrecognized element to the architectures that can be built from DNA oligonucleotides. Any duplex with complementary single-strand overhangs should be expected to give rise to regular coiled-coil structures.

A program has been developed to determine the diffraction pattern given by partially ordered fibres formed by macromolecules with helical symmetry. It is particularly useful for visualizing the splitting of layer lines typical of coiled coils. The program produces as output the diffraction diagram calculated for helices that are oriented along their axis but are randomly oriented in other directions. The results can be numerically analyzed and also visualized on-screen. The program has been applied to the diffraction patterns given by DNA and protein coiled coils.

Xenopus laevis nucleoplasmin is a molecular chaperone that mediates sperm decondensation and nucleosome assembly. Nucleoplasmin has three acidic tracts (A1, A2 and A3) and until recent years the long polyglutamic tract A2 was thought to be the binding site for basic proteins. However, the latest publications in this field show that neither A2 nor A3 is indispensable for histone and sperm-specific protein binding. In this work, we show that the mutation of only four acidic amino acid residues of the small A1 tract drastically reduces nucleoplasmin decondensing activity, pointing out this region as the potential binding site for sperm proteins.

The deoxyoligonucleotide d(CGCAATTGCG) has previously been crystallized in four different space groups. The crystals diffract to moderate resolution (2.3-2.9 A). Here, a fifth crystal form that diffracts to higher resolution (1.6 A) is presented which was obtained thanks to the use of Co2+ and cryogenic temperatures. The availability of five different crystal structures allows a thorough analysis of the conformational variability of this DNA sequence. It is concluded that the central hexamer sequence CAATTG has a practically constant conformation under all conditions, whilst the terminal base pairs at both ends vary considerably as a result of differing interactions in the crystals. The new crystal structure presented here is stabilized by guanine-Co2+-guanine interactions and the formation of C1+ -G8.C3 triplexes between neighbouring duplexes. As a result of the higher resolution of the crystal structure, a more regular structure was obtained and a clear definition of the spine of hydration was observed which was not visible in the four previous structures.

We have analyzed the relative orientation of basic amino acid side chains towards DNA in the nucleosome core particle. The electric field created by DNA phosphates has no apparent preferential orientation: no favored orientation of the arginine guanidinium group is found. Arginine may be either directly hydrogen bonded to a phosphate oxygen or stabilized in the minor groove by van der Waals contacts and the local negative electric field. On the other hand, the phosphate oxygen atoms hydrogen bonded to arginines are always found close to the plane defined by the guanidinium group. Thus it can be concluded that the interactions of arginine are strongly directional, those of phosphate are not. We also find that a highly charged fragment of histone H2B, which is placed between two DNA turns, has a very variable conformation. An increase in protein positive charge density apparently allows multiple nonspecific protein conformations when interacting with DNA.

▪ Abstract Cations are bound to nucleic acids in a solvated state. High-resolution X-ray diffraction studies of oligonucleotides provide a detailed view of Mg2+, and occasionally other ions bound to DNA. In a survey of several such structures, certain general observations emerge. First, cations bind preferentially to the guanine base in the major groove or to phosphate group oxygen atoms. Second, cations interact with DNA most frequently via water molecules in their primary solvation shell, direct ion-DNA contacts being only rarely observed. Thus, the solvated ions should be viewed as hydrogen bond donors in addition to point charges. Finally, ion interaction sites are readily exchangeable: The same site may be occupied by any ion, including spermine, as well as by a water molecule.

A theoretical investigation of the influence of pentahydrated Mg2+ cation binding on the Hoogsteen G−G·C triplet motif is presented. Theoretical calculations have been performed by using the structural data of the G−G·C triplet found in the crystallographic structure of the B-DNA duplex d(GCGAATTCG) [J. Biol. Chem. 1999, 274, 23683−23686]. The investigation involves a variety of theoretical analysis, which includes analysis of geometrical, energetic, electron topology, and electrostatic potential properties. Calculations show the large stabilizing effect of the Mg2+ cation on the G−G·C triplet. Such a stabilization mainly arises from the balance of electrostatic interactions between the cation with the three bases in the triplet. However, the cation-induced polarization, which is largely concentrated in the Hoogsteen guanine, also plays a significant contribution to the stabilization of the triplet. The results can be valuable to gain further insight into the cation-induced enhancement of the stability of...

We present the crystal structure of an acridine drug derivatized at carbon 9, [N(alpha)-(9-acridinoyl)-tetraarginine], intercalated within the dodecamer [d(CGCGAATTCGCG)](2). The presence of a lateral chain at the central carbon 9 atom differentiates this compound from most acridine drugs hitherto studied, which are usually derivatized at carbon 4. The DNA:drug interaction we observe differs from that observed in previous studies, which primarily involves shorter, mainly hexameric sequences, in two important regards: the acridine intercalates within an AA/TT base step, rather than within a CG/CG base step; and the binding site is located at the center of the sequence, rather than at one end of the duplex. In addition, we observe a novel crystal packing arrangement, with six dodecamer duplexes and seven hydrated magnesium ions in the asymmetric unit of a large (66.5 x 68.4 x 77.4 A(3)) unit cell in space group P2(1)2(1)2(1). The duplexes are organized in layers parallel to the ab plane, with consecutive layers crossing each other at right angles.

Different recombinant forms of nucleoplasmin including truncations at the carboxyl-terminal end of the molecule (r-NP121, r-NP142) have been expressed and purified. All of them were found to oligomerize, forming pentameric complexes which, according to their hydrodynamic properties, can be modeled by oblate ellipsoids of constant width. In this model, the highly charged carboxyl ends appear to be arranged around a pentameric core along the plane defined by the major axes of the ellipsoid. Importantly, all the recombinant forms appear to be able to decondense protamine-containing sperm nuclei. However, although the stoichiometry with which protamines bind to these forms appears to be constant (2.5 mol of protamine/mol of nucleoplasmin pentamer), the efficiency with which they remove protamines from the sperm DNA decreases in the following order: o-NPr-NP142or = r-NPr-NP121. Therefore, the main polyglutamic tract of nucleoplasmin (which is absent in r-NP121), while enhancing the efficiency of protamine removal, is not indispensable for sperm chromatin decondensation in the biological model we have used.

We have synthesized and crystallized in the presence of Zn(2+) ions the peptidyl-oligonucleotide adduct CH(3)CO-(Arg)(4)-NH-(CH(2))(6)-NH-p-d(CGCAATTGCG). This is the first structure obtained from a deoxyoligonucleotide crystallized in the presence of zinc ions. Zn ions are clearly visible in the 2.9 A resolution map. On the other hand, the peptide tail is not visible in the crystal structure as determined by X-ray diffraction. The terminal bases C1 and G10 are found in extra-helical positions. Their phosphates are ligands of a Zn(2+) ion, located in a special position of the unit cell. This ion plays an important role in the packing arrangement, since it binds four different DNA molecules. Two other Zn(2+) ions are also important for DNA packing. They interact specifically with the N7 atoms of the terminal G2 and G10 bases, but not with the internal G8. This result supports the hypothesis that transition metals do not interact with the bases of duplex DNA in the B form.

In this work, we explore the influence of different solvents and ions on the crystallization behavior of an all-AT dodecamer d(AATAAATTTATT)2 In all cases, the oligonucleotides are found as continuous columns of stacked duplexes. The spatial organization of such columns is variable; consequently we have obtained seven different crystal forms. The duplexes can be made to crystallize in either parallel or crossed columns. Such versatility in the formation of a variety of crystal forms is characteristic for this sequence. It had not been previously reported for any other sequence. In all cases, the oligonucleotide duplexes have been found to crystallize in the B form. The crystallization conditions determine the organization of the crystal, although no clear local interactions have been detected. Mg(2+) and Ni(2+) can be used in order to obtain compact crossed structures. DNA-DNA interactions in the crystals of our all-AT duplexes present crossovers which are different from those previously reported for mixed sequence oligonucleotides. Our results demonstrate that changes in the ionic atmosphere and the crystallization solvent have a strong influence on the DNA-DNA interactions. Similar ionic changes will certainly influence the biological activity of DNA. Modulation of the crystal structure by ions should also be explored in DNA crystal engineering. Liquid crystals with a peculiar macroscopic shape have also been observed.

The DNA of several pathogens is very rich in AT base pairs. Typical examples include the malaria parasite Plasmodium falciparum and the causative agents of trichomoniasis and trypanosomiases. This fact has prompted studies of drugs which interact with the minor groove of DNA, some of which are used in medical practice. Previous studies have been performed almost exclusively with the AATT sequence. New features should be uncovered through the study of different DNA sequences. In this paper, the crystal structure of the complex of the DNA duplex d(AAAATTTT)2 with the dicationic drug 4,4'-bis(imidazolinylamino) diphenylamine (CD27) is presented. The drug binds to the minor groove of DNA as expected, but it shows two new features that have not previously been described: (i) the drugs protrude from the DNA and interact with neighbouring molecules, so that they may act as cross-linking agents, and (ii) the drugs completely cover the whole minor groove of DNA and displace bound water. Thus, they may prevent the access to DNA of proteins such as AT-hook proteins. These features are also expected for other minor-groove binding drugs when associated with all-AT DNA. These findings allow a better understanding of this family of compounds and will help in the development of new, more effective drugs. New data on the biological interaction of CD27 with the causative agent of trichomoniasis, Trichomonas vaginalis, are also reported. © 2014 International Union of Crystallography.

Linear poly(amino-s-triazine)s with segments of ten and twelve methylene groups have been synthesized and characterized. Their structure has been studied by means of X-ray and electron diffraction, and molecular simulation. The polymers are found to crystallize in a unit cell very similar in chain axis projection to that of polyethylene. Furthermore, molecular simulation indicates that the polymethylene segments of the polymer chains are placed with a setting angle very close to that found in the polyethylene structure. The packing of the new polymers appears to be further stabilized by intermolecular hydrogen bonds that involve all nitrogen atoms of the amino-triazine units.

The aim of this study was to clarify whether Ni2+ ions could bind to guanine bases in a standard B-DNA duplex and eventually induce a B--Z transition. We have determined by X-ray crystallography at 3.1 A resolution the structure of the alternating deoxynucleotide d(CGTGTACACG), which contains both internal and terminal guanines. The duplex is in the B form. It is shown that nickel ions bind selectively to the N7 atom of guanine 10, which is in an extra-helical position, and guanine 2, which is in the terminal position of the duplex. It does not bind to guanine 4, which lies within a standard B-DNA tract. This simple but unambiguous result proves that nickel ions select between different guanines via steric accessibility. Guanine-Ni2+-guanine bridges among symmetry-related duplexes have also been found. These bridges may explain why Ni2+ ions may act either as a precipitant or a renaturing agent for DNA under certain conditions. The biochemical interaction of nickel with DNA can thus be related to its capacity to specifically bind to B-DNA regions with exposed guanines. Also, from the structural point of view, we have found a terminal cytosine, which forms a C.G:C reverse-Hoogsteen triple structure with a base pair of a neighbor duplex. This type of triplet is seldom found and is here described for the first time for a DNA structure.

We have studied by single-crystal X-ray diffraction the structure of a cyclic trimer model of nylon 11 (IUPAC name: 1,13,25-triazacyclohexatriacontane-2,14,26-trione). The crystal is organized in layers of hydrogen-bonded rings similar to nylon lamellar crystals. The rings fold both at the amide and methylene regions. Neighbor layers show van der Waals contacts among similar turns, either methylene/methylene or amide/amide. A new type of fold has been found, which includes an amide group and a methylene unit. Such folds may coexist in nylon lamellar crystals with conventional turns similar to the β-turn found in proteins. The orientation of up and down chains in the crystal is different from the orientation usually assumed for nylon 11, which suggests an alternative unit cell for nylon 11. We also include an analysis of the vertical displacement of hydrogen bonded layers found by different authors in nylon 11, which shows that it may vary between zero and three methylene units. In the trimer crystal we f...

We have determined by X-ray crystallography the structure of the hexamer duplex d(GGCGCC)2 in the A-form using ethanol as a precipitant. The same sequence had previously been crystallized in the B-form, but with 2-methyl-2,4-pentanediol as a precipitant. It appears that ethanol precipitation is a useful method to induce the formation of A-form crystals of DNA. Packing of the molecules in the crystal has unique features: the known interaction of A-DNA duplexes between terminal base-pairs and the minor groove of neighbor molecules is combined with a superstructure consisting in an alternation of DNA layers and solvent layers (water/ions). This organization in layers has been observed before, also with hexamers in the A conformation which crystallize in the same space group (C2221). The solvent layer has a precise thickness, although very few ordered water molecules can be detected. Another feature of this crystal is its large unit cell, which gives rise to an asymmetric unit with three hexamer duplexes. One of the three duplexes is quite different from the other two in several aspects: the number of base pairs per turn, the twist pattern, the mean value of the twist angle and the fact that one terminal base-pair is not stacked as part of the duplex and appears to be disordered. So the variability in conformation of this sequence is remarkable.

We describe the crystal structure of d(GCGAATTCG) determined by x-ray diffraction at atomic resolution level (0.89 A). The duplex structure is practically identical to that described at 2.05 A resolution (Van Meervelt, L., Vlieghe, D., Dautant, A., Gallois, B., Precigoux, G., and Kennard, O. (1995) Nature 374, 742-744), however about half of the phosphate groups show multiple conformations. The crystal has three regions with different solvent structure. One of them contains several ordered Mg(+2) ions and can be considered as an ionic crystal. A second region is formed by a network of ordered water molecules with a polygonal organization that binds three duplexes. The third region is formed by channels of solvent in which very few ordered solvent molecules are visible. The less ordered phosphates are found facing this channel. The latter region provides a view of DNA with highly movable charges, both negative phosphates and counterions, without a precise location.

Aliphatic amides are often used in the synthesis of peptidomimetic compounds. Here we present the structure of two cyclic dimers of aminoundecanoic acid as determined by x-ray diffraction. Each dimer contains two peptide groups and twenty methylene units. In one of the crystal structures, water is associated with the peptide groups, forming a chain of hexagons similar to those found in crambin, and in other protein and nucleic acid crystals. The aminoundecanoic rings show a fold at the peptide group, similar to either beta-turn type III or V found in proteins. Such folds are an adequate model for the peptide bond structure in nylon crystallites and peptidomimetic compounds.

We present the structure of the decanucleotide d(CGTATATACG) determined by single crystal X-ray diffraction at 1.58 A resolution. A netropsin drug is found in the minor groove with guanine stacked on a pyrrole ring of the drug, a feature described here for the first time. The stacked guanine is an extra-helical base coming from the end of a neighbour oligonucleotide. This observation may open the way to the development of minor groove binding drugs with a higher sequence selectivity. The oligonucleotide is in the B-conformation, but the terminal base-pairs are disrupted: the cytosine residues are disordered while the guanine residues penetrate into the minor groove of neighbouring duplexes. Four hydrated Ni ions with octahedral co-ordination are found associated with the N7 atoms of each guanine. The high affinity of these ions with guanine suggests that they may be used as probes for specific guanine residues.