Inherent DNA curvature and flexibility correlate with TATA box functionality

Vascular alteration in relation to fosfomycine: In silico and in vivo investigations using a chick embryo model

Fosfomycin residues are found in the egg following administration in the layer hen. In this regard, some aspects of embryo-toxicity of fosfomycin have been documented previously. The exact mechanism by which fosfomycin causes embryo-toxicity is not clearly understood. We hypothesis that fosfomycin may alter vasculature as well as normal expression of genes, which are associated with vascular development. Therefore, the present study aimed to address these issues through in silico and in vivo investigations. At first, embryo-toxicity and anti-angiogenic effects of fosfomycin were tested using computerized programs. After that, fertile chicken eggs were treated with fosfomycin and chorioallantoic membrane vasculature was assessed through morphometric, molecular and histopathological assays. The results showed that fosfomycin not only interacted with VEGF-A protein and promoter, but also altered embryonic vasculature and decreased expression level of VEGF-A. Reticulin staining of treated group was also confirmed decreased vasculature. The minor groove of DNA was the preferential binding site for fosfomycin with its selective binding to GC-rich sequences. We suggested that the affinity of fosfomycin for VEGF-A protein and promoter as well as alteration of the angiogenic signaling pathway may cause vascular damage during embryonic growth. Hence, veterinarians should be aware of such effects and limit the use of this drug during the developmental stages of the embryo, particularly in breeder farms. Considering the anti-angiogenic activity and sequence selectivity of fosfomycin, a major advantage that seems to be very promising is the fact that it is possible to achieve a sequence-selective binding drug for cancer.

Differential structure of the intronic promoter of the Bombyx mori A3 actin gene correlated with silkworm sensitivity/resistance to nucleopolyhedrovirus

Previous reports demonstrated that actin is necessary for nucleocapsid transport and viral gene expression during nucleopolyhedrovirus infection of Bombyx mori. The first intron of B. mori A3 actin contains a cryptic promoter that drives expression of a rare isoform. We detected differences in the size and nucleotide composition of the first intron of the A3 actin gene from B. mori strain C24A, which is more resistant to nucleopolyhedrovirus than the M11A strain (22 and 95% lethality, respectively). We sought to determine if resistance to BmMNPV infection and the A3 actin promoter structure are correlated. Intrinsically bent DNA sites in these sequences, which determine curved structures, were analyzed by electrophoretic mobility assays and the helical parameters ENDS ratio, roll and twist. We found both fragments to have non-centralized bent DNA sites with distinct ENDS ratio values, nucleotide positions and two-dimensional structures. Additionally, a conformational-sensitive gel electrophoresis assay identified an allelic variation found in strain M11A that is absent in strain C24A. These data suggest that A3 actin intronic sequence variations impair virus propagation and are markers of BmMNPV-resistant populations.

Replication origins oriGNAI3 and oriB of the mammalian AMPD2 locus nested in a region of straight DNA flanked by intrinsically bent DNA sites

The aim of this work was to determine whether intrinsically bent DNA sites are present at, or close to, the mammalian replication origins oriGNAI3 and oriB in the Chinese hamster AMPD2 locus. Using an electrophoretic mobility shift assay and in silico analysis, we located four intrinsically bent DNA sites (b1 to b4) in a fragment that contains the oriGNAI3 and one site (b5) proximal to oriB. The helical parameters show that each bent DNA site is curved in a left-handed superhelical writhe. A 2D projection of 3D fragment trajectories revealed that oriGNAI3 is located in a relatively straight segment flanked by bent sites b1 and b2, which map in previously identified Scaffold/Matrix Attachment Region. Sites b3 and b4 are located approximately 2 kb downstream and force the fragment into a strong closed loop structure. The b5 site is also located in an S/MAR that is found just downstream of oriB.

Docking studies on DNA-ligand interactions: building and application of a protocol to identify the binding mode

Despite DNA being an important target for several drugs, most of the docking programs are validated only for proteins and their ligands. In this paper, we used AutoDock 4.0 to perform self-dockings and cross dockings between two DNA ligands (a minor groove binder and an intercalator) and four distinct receptors: 1) crystallographic DNA without intercalation gap; 2) crystallographic DNA with intercalation gap; 3) canonical B-DNA; and 4) modified B-DNA with intercalation gap. Besides being efficient in self-dockings, AutoDock is capable of correctly identifying two of the main DNA binding modes with the condition that the target possesses an artificial intercalation gap. Therefore, we suggest a default protocol to identify DNA binding modes which uses a modified canonical DNA (with gap) as receptor. This protocol was applied to dock two different Troger bases to DNA and the predicted binding modes agree with those suggested, yet not established, by experimental data. We also applied the protocol to dock aflatoxin B(1) exo-8,9-epoxide, and the results are in complete agreement with experimental data from the literature. We propose that this approach can be used to investigate other ligands whose binding mode to DNA remains unknown, yielding a suitable starting point for further theoretical studies such as molecular dynamics simulations.

Intrinsic bent DNA colocalizes with the sequence involved in the Nd-sD mutation in the Bombyx mori fibroin light chain gene

Multiple sequence alignments of the Bombyx mori fibroin light chain gene (fib-L) from hybrids and from Chinese and Japanese strains demonstrated that 51.6% of the fib-L third intron is conserved. One of these conserved segments, 41 bp long, contains the sequence CGTTATTATACATATT, which is duplicated in the B. mori Nd-s(D) mutant. In the present work, electrophoretic mobility assays and computational analyses revealed a major peak of intrinsic bent DNA within the segment that undergoes breakage in the previously-described Nd-s(D) mutation. This result suggested that this intrinsically-curved region might mediate DNA cleavage and enhance recombination events in the third intron of the Bombyx mori fib-L gene.

Molecular dynamics of a kappaB DNA element: base flipping via cross-strand intercalative stacking in a microsecond-scale simulation

The sequence-dependent structural variability and conformational dynamics of DNA play pivotal roles in many biological milieus, such as in the site-specific binding of transcription factors to target regulatory elements. To better understand DNA structure, function, and dynamics in general, and protein···DNA recognition in the ‘κB’ family of genetic regulatory elements in particular, we performed molecular dynamics simulations of a 20-bp DNA encompassing a cognate κB site recognized by the proto-oncogenic ‘c-Rel’ subfamily of NF-κB transcription factors. Simulations of the κB DNA in explicit water were extended to microsecond duration, providing a broad, atomically detailed glimpse into the structural and dynamical behavior of double helical DNA over many timescales. Of particular note, novel (and structurally plausible) conformations of DNA developed only at the long times sampled in this simulation—including a peculiar state arising at ≈0.7 μs and characterized by cross-strand intercalative stacking of nucleotides within a longitudinally sheared base pair, followed (at ≈1 μs) by spontaneous base flipping of a neighboring thymine within the A-rich duplex. Results and predictions from the microsecond-scale simulation include implications for a dynamical NF-κB recognition motif, and are amenable to testing and further exploration via specific experimental approaches that are suggested herein.

p53-Induced DNA bending: the interplay between p53-DNA and p53-p53 interactions

Specific p53 binding-induced DNA bending and its underlying responsible forces are crucial for the understanding of selective transcription activation. Diverse p53-response elements exist in the genome; however, it is not known what determines the DNA bending and to what extent. In order to gain knowledge of the forces that govern the DNA bending, molecular dynamics simulations were performed on a series of p53 core domain tetramer-DNA complexes in which each p53 core domain was bound to a DNA quarter site specifically. By varying the sequence of the central 4-base pairs of each half-site, different DNA bending extents were observed. The analysis showed that the dimer-dimer interactions in p53 were similar for the complexes; on the other hand, the specific interactions between the p53 and DNA, including the interactions of Arg280, Lys120, and Arg248 with the DNA, varied more significantly. In particular, the Arg280 interactions were better maintained in the complex with the CATG-containing DNA sequence and were mostly lost in the complex with the CTAG-containing DNA sequence. Structural analysis shows that the base pairings for the CATG sequence were stable throughout the simulation trajectory, whereas those for the CTAG sequence were partially dissociated in part of the trajectory, which affected the stability of the nearby Arg280-Gua base interactions. Thus, DNA bending depends on the balance between the p53 dimer-dimer interactions and p53-DNA interactions, which is in turn related to the DNA sequence and DNA flexibility.

DNA bending in the replication zone of the C3 DNA puff amplicon of Rhynchosciara americana (Diptera: Sciaridae)

Intrinsic bent DNA sites were identified in the 4289 bp segment encompassing the replication zone which directs DNA amplification and transcription of the C3-22 gene of Rhynchosciara americana. Restriction fragments showed reduced electrophoretic mobility in polyacrylamide gels. The 2D modeling of the 3D DNA path and the ENDS ratio values obtained from the dinucleotide wedge model of Trifonov revealed the presence of four major bent sites, positioned at nucleotides -6753, -5433, -5133 and -4757. Sequence analysis showed that these bends are composed of 2-6 bp dA.dT tracts in phase with the DNA helical repeat. The circular permutation analysis permitted the verification that the fragments containing the bending sites promote curvature in other sequence contexts. Computer analyses of the 4289 bp sequence revealed low helical stability (DeltaG values), negative roll angles indicating a narrow minor groove and a putative matrix attachment region. The data presented in this paper add to information about the structural features involved in this amplified segment.

TBP binding capacity of the TATA box is associated with specific structural properties: AFM study of the IL-2R alpha gene promoter

DNA is not only a nucleotide sequence which allows the binding of regulators but its intrinsic structural properties such as curvature and flexibility are also viewed as playing an active role in the regulation of transcription. Our combination of computer modelling and AFM imaging allow direct access to DNA curvature and flexibility. We have searched for these DNA structural features involved in transcription regulation within the IL-2Ralpha gene promoter. Investigation of these structural characteristics shows concordant results. First, in the core promoter, the region containing the functional TATA box shows intrinsic curvature associated with a peculiar distribution of flexibility. Both these inherent properties are characteristic of this region as compared with the other parts of the promoter. Second, the proximal promoter exhibits two important regions: a first one flexible and curved, followed by a segment of rigid linear DNA, each localised within one of the two Positive Regulatory Regions PRRI and PRRII respectively. Based on these observations, we propose different roles for DNA curvature and/or flexibility in promoter sequences.

Scaffold/Matrix Attachment Regions and intrinsic DNA curvature

Recent approaches have failed to detect nucleotide sequence motifs in Scaffold/Matrix Attachment Regions (S/MARs). The lack of any known motifs, together with the confirmation that some S/MARs are not associated to any peculiar sequence, indicates that some structural elements, such as DNA curvature, have a role in chromatin organization and on their efficiency in protein binding. Similar to DNA curvature, S/MARs are located close to promoters, replication origins, and multiple nuclear processes like recombination and breakpoint sites. The chromatin structure in these regulatory regions is important to chromosome organization for accurate regulation of nuclear processes. In this article we review the biological importance of the co-localization between bent DNA sites and S/MARs.

A statistical approach for analyzing structural and regulative information in prokaryotic genomes

Although DNA is iconized as a straight double helix, it does not exist in this canonical form in biological systems. Instead, it is characterized by sequence dependent structural and dynamic deviations from the monotonous regularity of the canonical B-DNA. Despite the complexity of the system, we showed that DNA structural and dynamics large-scale properties can be predicted starting from the simple knowledge of nucleotide sequence by adopting a statistical approach. The paper reports the statistical analysis of large pools of different prokaryotic genes in terms of the sequence-dependent curvature and flexibility. Conserved features characterize the regions close to the Start Translation Site, which are related to their function in the regulation system. In addition, regular patterns with three-fold periodicity were found in the coding regions. They were reproduced in terms of the nucleotide frequency expected on the basis of the genetic code and the pertinent occurrence of the aminoacid residues.

Sequence-dependent solution structure and motions of 13 TATA/TBP (TATA-box binding protein) complexes

The TATA element is a well-known example of a DNA promoter sequence recognized by the TATA box binding protein (TBP) through its intrinsic motion and deformability. Although TBP recognizes the TATA element octamer unusually (through the minor groove, which lacks the distinctive features of the major groove), single base-pair replacements alter transcriptional activity. Recent crystallographic experiments have suggested that TATA/TBP complexes differing by a single base pair retain substantial structural similarity despite their functional differences in activating transcription. To investigate the subtle role of sequence-dependent motion within the TATA element and certain aspects of its effect on assembly of the transcriptional complex, we examine 5-ns dynamics trajectories of 13 variant TATA/TBP complexes differing from each other by a single base pair. They include the wild-type (WT) adenovirus 2 major late promoter (AdMLP) TATA element, TATAAAAG (the octamer specifies positions -31 to -24 with respect to the transcription initiation site), and the variants A31 (i.e., AATAAAAG), T30, A29, C29, G28, T28, T27, G26, T26, C25, T25, and T24. Our simulated TATA/TBP complexes develop sequence-dependent structure and motion trends that may lead to favorable orientations for high-activity variants (with respect to binding TFIIA, TFIIB, and other transcription factors), while conversely, accelerate dissociation of low-activity TATA/TBP complexes. The motions that promote favorable geometries for preinitiation complexes include small rotations between TBP's N- and C-terminal domains, sense strand DNA backbone "slithering," and rotations in TBP's H2 and H2' helices. Low-activity variants tend to translate the H1 and H1' helices and withdraw the intercalating phenylalanines. These cumulative DNA and protein motions lead to a spatial spread of complex orientations up to 4 A; this is associated with an overall bend of the variant TATA/TBP complexes that spans 93 degrees to 110 degrees (107 degrees for the crystal reference). Taken together, our analyses imply larger differences when these local structural and bending changes are extended to longer DNA (upstream and downstream) and suggest that specific local TATA/TBP motions (e.g., shifts in TBP helices and TATA bases and backbone) play a role in modulating the formation and maintenance of the transcription initiation complex.

Steady-state and time-resolved fluorescence studies indicate an unusual conformation of 2-aminopurine within ATAT and TATA duplex DNA sequences

2-Aminopurine (2-AP), a fluorescent analog of adenine, has been widely used as a probe for local DNA conformation, since excitation and emission characteristics and fluorescence lifetimes of 2-AP vary in a sequence-dependent manner within DNA. Using steady-state and time-resolved fluorescence techniques, we report that 2-AP appears to be unusually stacked in the internal positions of ATAT and TATA in duplex DNA. The excitation wavelength maxima for 2-AP within these contexts were red shifted, indicating reduced solvent exposure for the fluorophore. Furthermore, in these contexts, 2-AP fluorescence was resistant to acrylamide-dependent collisional quenching, suggesting that the fluorophore is protected by its stacked position within the duplex. This conclusion was further reinforced by the presence of a secondary peak at 275 nm in the fluorescence excitation spectra that is indicative of efficient excitation energy transfer from nearby non-fluorescent DNA bases. Fluorescence anisotropy decay and internal angular 'wobbling' motion measurements of 2-AP within these alternating AT contexts were also consistent with the fluorophore being highly constrained and immobile within the base stack. When these fluorescence characteristics are compared with those of 2-AP within other duplex DNA sequence contexts, they are unique.

Time-resolved fluorescence resonance energy transfer studies of DNA bending in double-stranded oligonucleotides and in DNA-protein complexes

Time-resolved Förster resonance energy transfer (trFRET) has been used to obtain interdye distance distributions. These distributions give the most probable distance as well as a parameter, sigma, that characterize the width of the distribution. This latter parameter contains information not only on the flexibility of the dyes tethered to macromolecules, but on the flexibility of the macromolecules. Both the most probable interdye distance as well as sigma provide insight into DNA static bending and DNA flexibility. Time-resolved fluorescence anisotropy and static anisotropy measurements can be combined to provide a measure of the cone angle within which the tethered dyes appear to wobble. When this motion is an order of magnitude faster than the average lifetime that characterizes transfer, an average value of the dipolar orientational parameter kappa2 can be calculated for various mutual dye orientations. The resulting kappa2 distribution is very much narrower than the limiting values of 0 and 4, allowing more precise distances and distance changes to be determined. Static and time-resolved fluorescence data can be combined to constrain the analyses of DNA-protein kinetics to provide thermodynamic parameters for binding and for conformational changes along a reaction coordinate. The parameter sigma can be used to model multiple DNA-protein complexes with varying DNA bend angles in a global fitting of trFRET data. Such a global fitting approach has shown how the range of bends in single base DNA variants, when bound by the TATA binding protein (TBP), can be understood in terms of two limiting forms. Time-resolved FRET, combined with steady-state FRET, can be used to show not only how osmolytes affect the binding of DNA to proteins, but also how DNA bending depends on osmolyte concentration in the DNA-protein complexes.

Comparison of TATA-binding protein recognition of a variant and consensus DNA promoters

Assembly of transcription pre-initiation complexes proceeds from the initial complex formed between "TATA" bearing promoter DNA and the TATA-binding protein (TBP). Our laboratory has been investigating the relationships among TATA sequence, TBP center dot TATA solution structure, recognition mechanisms, and transcription efficiency. TBP center dot TATA interactions have been modeled by global analysis of detailed kinetic and thermodynamic data obtained using fluorimetric and fluorometric techniques in conjunction with fluorescence resonance energy transfer. We have reported recently that TBP recognition of two consensus promoters, adenovirus major late (AdMLP: TATAAAAG) and E4 (TATATATA), is well described by a linear two-intermediate mechanism with simultaneous DNA binding and bending. Similar DNA geometries and high transcription efficiencies characterize these TBP x TATA complexes. Here we show that, in contrast to the consensus sequences, TBP recognition of a variant sequence (C7: TATAAACG) is described by a three-step model with two branching pathways. One pathway proceeds through an intermediate having severely bent DNA, reminiscent of the consensus interactions, with the other branch yielding a unique conformer with shallowly bent DNA. The resulting TBP x C7 complex has a dramatically different solution conformation than for TBP x DNA(CONSENSUS) and is correlated with diminished relative transcription activity. The temperature dependence of the TBP x C7 helical bend is postulated to derive from population shifts between the conformers with slightly and severely bent DNA.

ADAPT: a molecular mechanics approach for studying the structural properties of long DNA sequences

We describe an original approach to determining sequence-structure relationships for DNA. This approach, termed ADAPT, combines all-atom molecular mechanics with a multicopy algorithm to build nucleotides that contain all four standard bases in variable proportions. These nucleotides enable us to search very rapidly for base sequences that energetically favor chosen types of DNA deformation or chosen DNA-protein or DNA-ligand interactions. Sequences satisfying the chosen criteria can be found by energy minimization, combinatorial sequence searching, or genome scanning, in a manner similar to the threading approaches developed for protein structure prediction. In the latter case, we are able to analyze roughly 2000 base pairs per second. Applications of the method to DNA allomorphic transitions, DNA deformation, and specific DNA interactions are presented.

Molecular dynamics studies of trinucleotide repeat DNA involved in neurodegenerative disorders

Expansion of trinucleotide repeat DNA of the classes CAG-CTG, CGG-CCG and GAA-TTC are found to be associated with several neurodegenerative disorders. Different mechanisms have been attributed to the expansion of triplets, mainly involving the formation of alternate secondary structures by such repeats. This paper reports the molecular dynamics simulation of triplet repeat DNA sequences to study the basic structural features of DNA that are responsible for the formation of structures such as hairpins and slip-strand DNA leading to expansion. All the triplet repeat sequences studied were found to be more flexible compared to the control sequence unassociated with disease. Moreover, flexibility was found to be in the order CAG-CTG > CGG-CCG approximately GAA-TTC, the highly flexible CAG-CTG repeat being the most common cause of neurodegenerative disorders. In another simulation, a single G-C to T-A mutation at the 9th position of the CAG-CTG repeat exhibited a reduction in bending compared to the pure 15-mer CAG-CTG repeat. EPM1 dodecamer repeat associated with the pathogenesis of progressive myoclonus epilepsy was also simulated and showed flexible nature suggesting a similar expansion mechanism.

Marked stepwise differences within a common kinetic mechanism characterize TATA-binding protein interactions with two consensus promoters

Binding of the TATA-binding protein (TBP) to promoter DNA bearing the TATA sequence is an obligatory initial step in RNA polymerase II transcription initiation. The interactions of Saccharomyces cerevisiae TBP with the E4 (TATATATA) and adenovirus major late (TATAAAAG) promoters have been modeled via global analysis of kinetic and thermodynamic data obtained using fluorescence resonance energy transfer. A linear two-intermediate kinetic mechanism describes the reaction of both of these consensus strong promoters with TBP. Qualitative features common to both interactions include tightly bound TBP-DNA complexes with similar solution geometries, simultaneous DNA binding and bending, and the presence of intermediate TBP-DNA conformers at high mole fraction throughout most of the reaction and at equilibrium. Despite very similar energetic changes overall, the stepwise entropic and enthalpic compensations along the two pathways differ markedly following the initial binding/bending event. Furthermore, TBP-E4 dissociation ensues from both replacement and displacement processes, in contrast to replacement alone for TBP-adenovirus major late promoter. A model is proposed that explicitly correlates these similarities and differences with the sequence-specific structural properties inherent to each promoter. This detailed mechanistic comparison of two strong promoters interacting with TBP provides a foundation for subsequent comparison between consensus and variant promoter sequences reacting with TBP.

Mapping of intrinsic bent DNA sites in the upstream region of DNA puff BhC4-1 amplified gene

We have identified bent DNA sites in the distal and proximal DNA puff BhC4-1 amplified gene promoter region of Bradysia hygida. The 2D modeling of the 3D DNA path and the ENDS ratio values calculated in this promoter region resulted in the identification of ten pronounced bent sites named BhC4B - 9 to + 1. The 1847 bp fragment (- 3697 to - 1850) in relation to the transcription start site shows multiple bending sites, BhC4B - 9 to BhC4B - 4, with periodicity approximately 300 bp. The analysis of the other identified bent region, starting at position - 957, reveals that the BhC4B + 1 bent site colocalizes with the putative BhC4-1 minimal promoter. The sequence analysis of bent site BhC4B - 4 shows a distribution of dA*dT at approximately 10 bp intervals between the middle of each tract, but intervals with more than one turn, approximately 20 bp, two helix turns, were detected in the other bent sites described here. The bent sites BhC4B - 6 and BhC4B - 4, contain two consensus sequences, with 60 bp each. The apparent molecular weight of fragments in the BhC4-1 promoter region were estimated in agarose gels and compared with the data obtained in polyacrylamide gels without and with ethidium bromide. The mobility reduction ratios (R-values) were determined, and a high R-value, 1.80, for a 1215 bp fragment in the distal promoter region and a 1.23 significant R-value for a 662 bp fragment in the proximal segment were found. To further analyze the predicted bent DNA sites in these fragments, the 2D trajectories of the 3D DNA path and other parameters, AT percentage, roll angle, ENDS ratio and DeltaG, were determined. The role of these bent sites in the BhC4-1 transcription regulation is discussed.

Dynamic simulations of 13 TATA variants refine kinetic hypotheses of sequence/activity relationships 1 1Edited by B. Honig

The fundamental relationship between DNA sequence/deformability and biological function has attracted numerous experimental and theoretical studies. A classic prototype system used for such studies in eukaryotes is the complex between the TATA element transcriptional regulator and the TATA-box binding protein (TBP). The recent crystallographic study by Burley and co-workers demonstrated the remarkable structural similarity contrasted to different transcriptional activity of 11 TBP/DNA complexes in which the DNAs differed by single base-pairs. By simulating these TATA variants and two other single base-pair variants that were not crystallizable, we uncover sequence-dependent structural, energetic, and flexibility properties that tailor TATA elements to TBP interactions, complementing many previous studies by refining kinetic hypotheses on sequence/activity correlations. The factors that combine to produce favorable elements for TBP activity include overall flexibility; minor groove widening, as well as roll, rise, and shift increases at the ends of the TATA element; untwisting within the TATA element accompanied by large roll at the TATA element ends; and relatively low maximal water densities around the DNA. These features accompany the severe deformation induced by the minor-groove binding protein, which kinks the TATA element at the ends and displaces local water molecules to form stabilizing hydrophobic contacts. Interestingly, the preferred bending direction itself is not a significant predictor of activity disposition, although certain variants (such as wild-type AdMLP, 5'-TATA4G-3', and inactive A29, 5'-TA6G-3') exhibit large preferred bends in directions consistent with their activity or inactivity (major groove and minor groove bends, respectively). These structural, flexibility, and hydration preferences, identified here and connected to a new crystallographic study of a larger group of DNA variants than reported to date, highlight the profound influence of single base-pair DNA variations on DNA motion. Our refined kinetic hypothesis suggests the functional implications of these motions in a kinetic model of TATA/TBP recognition, inviting further theoretical and experimental research.

A detailed interpretation of OH radical footprints in a TBP-DNA complex reveals the role of dynamics in the mechanism of sequence-specific binding

The hydroxyl radical footprint of the TATA-binding protein (TBP) bound to the high-affinity sequence TATAAAAG of the adenovirus 2 major late promoter has been quantitatively compared to a 2 ns molecular dynamics simulation of the complex in aqueous solution at room temperature using the CHARMM23 potential. The nucleotide-by-nucleotide analysis of the TBP-TATA hydroxyl radical footprint correlates with the solvent-accessible surface calculated from the dynamics simulation. The results suggest that local reactivity towards OH radicals results from the interplay between the local DNA geometry imposed by TBP binding, and the dynamics of the side-chains contacting the sugar hydrogen atoms. Analysis of the dynamics suggests that, over time, TBP forms stable interactions with the sugar-phosphate backbone through multiple contacts to different partners. This mechanism results in an enthalpic advantage to complex formation at a low entropic cost.

Bending and adaptability to proteins of the cAMP DNA-responsive element: molecular dynamics contrasted with NMR

DNA bending is assumed to play a crucial role during recognition of the cAMP-responsive element (CRE) by transcription factors. However, diverging results have been obtained for the bending direction of the unbound double helix. The refined NMR structures present a bend directed toward the minor groove, while biochemical methods conclude that there is a bend toward the major groove. The present 10-ns molecular dynamics (MD) simulation of d(GAGATGACGTCATCTC)(2), which contains the octamer CRE in its center, was carried out with AMBER in explicit water and counterions. It shows that CRE is a flexible segment, although it is bent slightly toward the major groove (7 degrees -8 degrees ) on the average. The MD structure agrees with both the biochemical results and unrefined NMR data. The divergence with the NMR refined structures suggests an improper electrostatic parameterization in the refinement software. The malleability of the central CpG is certainly the major contribution to the curving of the whole CRE segment in both the unbound and bound states. Comparison with the crystal structure of CRE bound to GCN4 shows that the deformation induced by the protein is concentrated mainly on the CpG step, rendering the bound structure of CRE closer to the structure of the 12-0 tetradecanoylphorbol-beta-acetate-responsive element.

Nucleosome structural features and intrinsic properties of the TATAAACGCC repeat sequence

Nucleosomes, the fundamental building blocks of chromatin, play an architectural role in ensuring the integrity of the genome and act as a regulator of transcription. Intrinsic properties of the underlying DNA sequence, such as flexibility and intrinsic bending, direct the formation of nucleosomes. We have earlier identified genomic nucleosome-positioning sequences with increased in vitro ability for nucleosome formation. One group of sequences bearing a 10-base pair consensus repeat sequence of TATAAACGCC had the highest reported nucleosome affinity from genomic material. Here, we report the intrinsic physical properties of this sequence and the structural details of the nucleosome it forms, as analyzed by footprinting techniques. The minor groove is buried toward the histone octamer at the AA steps and facing outwards at the CC steps. By cyclization kinetics, the overall helical repeat of the free DNA sequence was found to be 10.5 base pairs/turn. Our experiments also showed that this sequence is highly flexible, having a J-factor 25-fold higher than that of random sequence DNA. In addition, the data suggest that twist flexibility is an important determinant for translational nucleosome positioning, particularly over the dyad region.

The biology of eukaryotic promoter prediction--a review

Computational prediction of eukaryotic promoters from the nucleotide sequence is one of the most attractive problems in sequence analysis today, but it is also a very difficult one. Thus, current methods predict in the order of one promoter per kilobase in human DNA, while the average distance between functional promoters has been estimated to be in the range of 30-40 kilobases. Although it is conceivable that some of these predicted promoters correspond to cryptic initiation sites that are used in vivo, it is likely that most are false positives. This suggests that it is important to carefully reconsider the biological data that forms the basis of current algorithms, and we here present a review of data that may be useful in this regard. The review covers the following topics: (1) basal transcription and core promoters, (2) activated transcription and transcription factor binding sites, (3) CpG islands and DNA methylation, (4) chromosomal structure and nucleosome modification, and (5) chromosomal domains and domain boundaries. We discuss the possible lessons that may be learned, especially with respect to the wealth of information about epigenetic regulation of transcription that has been appearing in recent years.