A Possible Role of DNA Superstructures in Genome Evolution

The concept of DNA as a simple repository of the gene information has changed in that of a polymorphic macromolecule, which plays a relevant part in the management of the complex biochemical transformations in living matter. As a consequence of the slight stereochemical differences between base pairs, the direction of the DNA double helix axis undergoes deterministic writhing. A useful representation of such sequence-dependent structural distortions is the curvature diagram. Here, it is reported as an evolution simulation obtained by extensive point mutations along a biologically important DNA tract. The curvature changes, consequence of the point mutations. were compared to the related experimental gel electrophoresis mobility. The curvature of most mutants decreases and the mobility increases accordingly, suggesting the curvature of that tract is genetically selected. Moreover, DNA images by scanning force microscopy, show evidence of a sequence-dependent adhesion of curved DNA tracts to inorganic crystal surfaces. In particular, mica shows a large affinity towards the TT-rich dinucleotide sequences. This suggests a possible mechanism of selection of curved DNA regions, characterized by AA.TT dinucleotides in phase with double-helical periodicity, in the very early evolution steps.     

Attraction, phasing and neighbour effects of histone octamers on curved DNA

Nucleosome core particles were reconstituted on various DNA fragments containing a Crithidia fasciculata kinetoplast curved tract. The results show that, on curved DNA, nucleosome core particles form six- to sevenfold preferentially, relative to bulk sequences. The preferential deposition occurs at multiple periodic positions, whose distribution reveals a unique rotational setting of DNA with respect to the histone octamer surface and whose average periodicity is 10.26 +/- 0.04. Evidence is provided for a context effect in histone octamer deposition: octamers bound to a segment of curved DNA influence the positions of neighbour octamers. Taken together, the preferential formation of nucleosome core particles and the influence on the localization of neighbouring particles suggest for intrinsically bent sequences the biologically relevant role of organizers of nucleosomal arrays.     

Characteristics of nucleosome core DNA and their applications in predicting nucleosome positions

By analyzing dinucleotide position-frequency data of yeast nucleosome-bound DNA sequences, dinucleotide periodicities of core DNA sequences were investigated. Within frequency domains, weakly bound dinucleotides (AA, AT, and the combinations AA-TT-TA and AA-TT-TA-AT) present doublet peaks in a periodicity range of 10-11 bp, and strongly bound dinucleotides present a single peak. A time-frequency analysis, based on wavelet transformation, indicated that weakly bound dinucleotides of core DNA sequences were spaced smaller (∼10.3 bp) at the two ends, with larger (∼11.1 bp) spacing in the middle section. The finding was supported by DNA curvature and was prevalent in all core DNA sequences. Therefore, three approaches were developed to predict nucleosome positions. After analyzing a 2200-bp DNA sequence, results indicated that the predictions were feasible; areas near protein-DNA binding sites resulted in periodicity profiles with irregular signals. The effects of five dinucleotide patterns were evaluated, indicating that the AA-TT pattern exhibited better performance. A chromosome-scale prediction demonstrated that periodicity profiles perform better than previously described, with up to 59% accuracy. Based on predictions, nucleosome distributions near the beginning and end of open reading frames were analyzed. Results indicated that the majority of open reading frames’ start and end sites were occupied by nucleosomes.     

Intrinsic curvature of DNA influences LacR-mediated looping

Protein-mediated DNA looping is a common mechanism for regulating gene expression. Loops occur when a protein binds to two operators on the same DNA molecule. The probability of looping is controlled, in part, by the basepair sequence of inter-operator DNA, which influences its structural properties. One structural property is the intrinsic or stress-free curvature. In this article, we explore the influence of sequence-dependent intrinsic curvature by exercising a computational rod model for the inter-operator DNA as applied to looping of the LacR-DNA complex. Starting with known sequences for the inter-operator DNA, we first compute the intrinsic curvature of the helical axis as input to the rod model. The crystal structure of the LacR (with bound operators) then defines the requisite boundary conditions needed for the dynamic rod model that predicts the energetics and topology of the intervening DNA loop. A major contribution of this model is its ability to predict a broad range of published experimental data for highly bent (designed) sequences. The model successfully predicts the loop topologies known from fluorescence resonance energy transfer measurements, the linking number distribution known from cyclization assays with the LacR-DNA complex, the relative loop stability known from competition assays, and the relative loop size known from gel mobility assays. In addition, the computations reveal that highly curved sequences tend to lower the energetic cost of loop formation, widen the energy distribution among stable and meta-stable looped states, and substantially alter loop topology. The inclusion of sequence-dependent intrinsic curvature also leads to nonuniform twist and necessitates consideration of eight distinct binding topologies from the known crystal structure of the LacR-DNA complex.     

Sequence-dependent deformational anisotropy of chromatin DNA.

As found in previous work (E.N. Trifonov and J.L. Sussman, Proc. Natl. Acad. Sci. USA, in press) some dinucleotides of the chromatin DNA sequences have a clear tendency to be repeated along the sequences with a period of about 10.5 bases. A special iteration procedure is developed to find if there are phase relationships between different periodically repeating dinucleotides of chromatin DNA. A very specific symmetrical pattern of preferences of different dinucleotides to certain positions within a repeating 10.5 base frame is indeed found. This is interpreted as a manifestation of sequence-dependent deformational anisotropy of the chromatin DNA which facilitates its smooth folding in chromatin. The pattern found can be used for locating unidirectionally curved portions of the DNA molecules, possibly corresponding to nucleosomal DNA. This implies that the DNA is bound to the nucleosomes by one specific side which corresponds to the direction of the sequence-dependent curving of the DNA axis. The 10.5 base periodicity found can be considered as the second message present in chromatin DNA sequences together with 3 base frame coding message.     

Signatures of Protein-DNA Recognition in Free DNA Binding Sites

One obstacle to achieving complete understanding of the principles underlying sequence-dependent recognition of DNA is the paucity of structural data for DNA recognition sequences in their free (unbound) state. Here, we carried out crystallization screening of 50 DNA duplexes containing cognate protein binding sites and obtained new crystal structures of free DNA binding sites for three distinct modes of DNA recognition: anti-parallel β strands (MetR), helix-turn-helix motif + hinge helices (PurR), and zinc fingers (Zif268). Structural changes between free and protein-bound DNA are manifested differently in each case. The new DNA structures reveal that distinctive sequence-dependent DNA geometry dominates recognition by MetR, protein-induced bending of DNA dictates recognition by PurR, and deformability of DNA along the A-B continuum is important in recognition by Zif268. Together, our findings show that crystal structures of free DNA binding sites provide new information about the nature of protein-DNA interactions and thus lend insights towards a structural code for DNA recognition.     

Indirect recognition in sequence-specific DNA binding by Escherichia coli integration host factor: the role of DNA deformation energy

Integration host factor (IHF) is a bacterial histone-like protein whose primary biological role is to condense the bacterial nucleoid and to constrain DNA supercoils. It does so by binding in a sequence-independent manner throughout the genome. However, unlike other structurally related bacterial histone-like proteins, IHF has evolved a sequence-dependent, high affinity DNA-binding motif. The high affinity binding sites are important for the regulation of a wide range of cellular processes. A remarkable feature of IHF is that it employs an indirect readout mechanism to bind and wrap DNA at both the nonspecific and high affinity (sequence-dependent) DNA sites. In this study we assessed the contributions of pre-formed and protein-induced DNA conformations to the energetics of IHF binding. Binding energies determined experimentally were compared with energies predicted for the IHF-induced deformation of the DNA helix (DNA deformation energy) in the IHF-DNA complex. Combinatorial sets of de novo DNA sequences were designed to systematically evaluate the influence of sequence-dependent structural characteristics of the conserved IHF recognition elements of the consensus DNA sequence. We show that IHF recognizes pre-formed conformational characteristics of the consensus DNA sequence at high affinity sites, whereas at all other sites relative affinity is determined by the deformational energy required for nearest-neighbor base pairs to adopt the DNA structure of the bound DNA-IHF complex.     

Distamycin A affects the stability of NF-kappaB p50-DNA complexes in a sequence-dependent manner

The effect of two different DNA minor groove binding molecules, Hoechst 33258 and distamycin A, on the binding kinetics of NF-kappaB p50 to three different specific DNA sequences was studied at various salt concentrations. Distamycin A was shown to significantly increase the dissociation rate constant of p50 from the sequences PRDII (5'-GGGAAATTCC-3') and Ig-kappa B (5'-GGGACTTTCC-3') but had a negligible effect on the dissociation from the palindromic target-kappaB binding site (5'-GGGAATTCCC-3'). By comparison, the effect of Hoechst 33258 on binding of p50 to each sequence was found to be minimal. The dissociation rates for the protein--DNA complexes increased at higher potassium chloride concentrations for the PRDII and Ig-kappaB binding motifs and this effect was magnified by distamycin A. In contrast, p50 bound to the palindromic target-kappaB site with a much higher intrinsic affinity and exhibited a significantly reduced salt dependence of binding over the ionic strength range studied, retaining a K(D) of less than 10 pM at 150 mM KCl. Our results demonstrate that the DNA binding kinetics of p50 and their salt dependence is strongly sequence-dependent and, in addition, that the binding of p50 to DNA can be influenced by the addition of minor groove-binding drugs in a sequence-dependent manner.     

DNA动力学柔性的统计力学模型

考虑碱基对之间的非紧邻相互作用、涨落的序列依赖效应和非对称涨落,提出了DNA构象的统计力学模型,给出了DNA柔性的新定义。作为模型的应用,对12种三核苷酸重复序列的动力学柔性作了预测。理论预测与其它方法得到的结论比较,有很好的一致性。对模型和结论的理论意义作了讨论。     

Sequence‐dependent modelling of local DNA bending phenomena: curvature prediction and vibrational analysis

Bending is a local conformational micropolymorphism of DNA in which the original B-DNA structure is only distorted but not extensively modified. Bending can be predicted by simple static geometry models as well as by a recently developed elastic model that incorporate sequence dependent anisotropic bendability (SDAB). The SDAB model qualitatively explains phenomena including affinity of protein binding, kinking, as well as sequence-dependent vibrational properties of DNA. The vibrational properties of DNA segments can be studied by finite element analysis of a model subjected to an initial bending moment. The frequency spectrum is obtained by applying Fourier analysis to the displacement values in the time domain. This analysis shows that the spectrum of the bending vibrations quite sensitively depends on the sequence, for example the spectrum of a curved sequence is characteristically different from the spectrum of straight sequence motifs of identical basepair composition. Curvature distributions are genome-specific, and pronounced differences are found between protein-coding and regulatory regions, respectively, that is, sites of extreme curvature and/or bendability are less frequent in protein-coding regions. A WWW server is set up for the prediction of curvature and generation of 3D models from DNA sequences (http://www.icgeb.trieste.it/dna).This revised version was published online in October 2005 with corrections to the Cover Date.     

Specificity in DNA recognition by a peptide from papillomavirus E2 protein

The E2 proteins of papillomavirus specifically bind to double-stranded DNA containing the consensus sequence ACCG-N4-CGGT, where N is any nucleotide. Here, we show the binding and recognition of dissimilar DNA sequences by an 18 amino-acid peptide (alpha1E2), which corresponds to the DNA-recognition helix, alpha-helix-1. Isothermal DNA binding assays performed with the DNA consensus sequence show saturable curves with alpha1E2 peptide, and the alpha1E2 peptide is converted to an ordered conformation upon complexation. Measurements performed with non-specific DNA sequence fail to saturate, a behavior characteristic of non-specific binding. Binding of the alpha1E2 peptide to these DNA sequences display a different counter-ion dependence, indicating a dissimilar, sequence-dependent mechanism of interaction. Quantitative stoichiometric measurements revealed the specificity in alpha1E2 peptide recognition of the ACCG half-site, demonstrating capacity for discrimination of nucleic acid bases sequences without the need of a whole protein architecture.     

DNA sequence-dependent contributions of core histone tails to nucleosome stability: differential effects of acetylation and proteolytic tail removal

Modulation of nucleosome stability in chromatin plays an important role in eukaryotic gene expression. The core histone N-terminal tail domains are believed to modulate the stability of wrapping nucleosomal DNA and the stability of the chromatin filament. We analyzed the contribution of the tail domains to the stability of nucleosomes containing selected DNA sequences that are intrinsically straight, curved, flexible, or inflexible. We find that the presence of the histone tail domains stabilizes nucleosomes containing DNA sequences that are intrinsically straight or curved. However, the tails do not significantly contribute to the free energy of nucleosome formation with flexible DNA. Interestingly, hyperacetylation of the core histone tail domains does not recapitulate the effect of tail removal by limited proteolysis with regard to nucleosome stability. We find that acetylation of the tails has the same minor effect on nucleosome stability for all the selected DNA sequences. A comparison of histone partitioning between long donor chromatin, acceptor DNA, and free histones in solution shows that the core histone tails mediate internucleosomal interactions within an H1-depleted chromatin fiber amounting to an average free energy of about 1 kcal/mol. Thus, such interactions would be significant with regard to the free energies of sequence-dependent nucleosome positioning. Last, we analyzed the contribution of the H2A/H2B dimers to nucleosome stability. We find that the intact nucleosome is stabilized by 900 cal/mol by the presence of the dimers regardless of sequence. The biological implications of these observations are discussed.     

Principles of sequence-dependent flexure of DNA

The curvature of a bent rod may be defined in several different, but equivalent ways. The best way of describing the curvature of double-helical DNA is by an angle of turning per base step. Curvature comes mainly from the angle of roll between successive base-pairs, and this is defined as positive when the angle opens up on the minor groove side of the bases. DNA forms a plane curve if the roll angle values along the molecule alternate periodically between positive and negative, with a complete period equal to the helical repeat. It is known from studies of crystallized oligomers that the roll angles for particular dinucleotide steps have preferred values, or lie in preferred ranges of values. Therefore the formation of a plane curve will be easier with some base sequences of DNA than with others. We set up a computer algorithm for determining the ease with which DNA of given sequence will adopt a curved form. The algorithm has two different sets of constants: in model 1 the base step parameters come from an inspection of crystallized oligomers, and in model 2 data from a statistical survey of the incidence of dinucleotide steps in a large number of samples of chicken erythrocyte core DNA is incorporated. Both forms of the algorithm successfully locate the dyad of the nucleosome sequence (modulo 10) in a frog gene, and suggest strongly that sequence-dependent flexural properties of DNA play a part in the recognition of binding sites by nucleosome cores.     

The main role of the sequence-dependent DNA elasticity in determining the free energy of nucleosome formation on telomeric DNAs

Using a competitive reconstitution assay, we measured the free energy spent in nucleosome formation of eight telomeric DNAs, differing in sequence and/or in length. The obtained values are in satisfactorily good agreement with those derived from a theoretical model that allows the calculation of the free energy of nucleosome formation on the basis of sequence-dependent DNA elasticity, using a statistical thermodynamic approach. Both theoretical and experimental evaluations show that telomeres are characterized by the highest free energies of nucleosome formation among all the DNA sequences so far studied. The free energy of nucleosome formation varies according to the different telomeric sequences and the length of the fragments. Theoretical analysis and experimental mapping by lambda exonuclease show that telomeric nucleosomes occupy multiple positions spaced every telomeric repeat. Sequence-dependent DNA elasticity appears as the main determinant of the stability of telomeric nucleosomes and their multiple translational positioning.     

DNA Sequence-Directed Organization of Chromatin: Structure-Based Computational Analysis of Nucleosome-Binding Sequences

The folding of DNA on the nucleosome core particle governs many fundamental issues in eukaryotic molecular biology. In this study, an updated set of sequence-dependent empirical “energy” functions, derived from the structures of other protein-bound DNA molecules, is used to investigate the extent to which the architecture of nucleosomal DNA is dictated by its underlying sequence. The potentials are used to estimate the cost of deforming a collection of sequences known to bind or resist uptake in nucleosomes along various left-handed superhelical pathways and to deduce the features of sequence contributing to a particular structural form. The deformation scores reflect the choice of template, the deviations of structural parameters at each step of the nucleosome-bound DNA from their intrinsic values, and the sequence-dependent “deformability” of a given dimer. The correspondence between the computed scores and binding propensities points to a subtle interplay between DNA sequence and nucleosomal folding, e.g., sequences with periodically spaced pyrimidine-purine steps deform at low cost along a kinked template whereas sequences that resist deformation prefer a smoother spatial pathway. Successful prediction of the known settings of some of the best-resolved nucleosome-positioning sequences, however, requires a template with “kink-and-slide” steps like those found in high-resolution nucleosome structures.     

Conserved DNA structures in origins of replication.

According to the model of Bramhill and Kornberg, initiation of DNA replication in prokaryotes involves binding of an initiator protein to origin DNA and subsequent duplex opening of adjacent direct repeat sequences. In this report, we have used computer analysis to examine the higher-order DNA structure of a variety of origins of replication from plasmids, phages, and bacteria in order to determine whether these sequences are localized in domains of altered structure. The results demonstrate that the primary sites of initiator protein binding lie in discrete domains of DNA bending, while the direct repeats lie within well-defined boundaries of an unusual anti-bent domain. The anti-bent structures arise from a periodicity of A3 and T3 tracts which avoids the 10-11 bp bending periodicity. Since DNA fragments which serve as replicators in yeast also contain these two conserved structural elements, the results provide new insight into the universal role of conserved DNA structures in DNA replication.     

CURVATURE: software for the analysis of curved DNA

Software is presented to plot the sequence-dependent spatialtrajectory of the DNA double helix and/or distribution of curvaturealong the DNA molecule. The nearest-neighbor wedge model isimplemented to calculate overall DNA path using local helixparameters: helix twist angle, wedge (deflection) angle anddirection (of deflection) angle. The procedures described provedto be very convenient as tools for investigation of a relationshipbetween overall DNA curvature and its gel electrophoretic mobility.All parameters of the model had been estimated from experimentaldata. Using these wedge parameters the program takes, as input,any DNA sequence and calculates the likely degree of curvatureat each point along the molecule. This information is displayedboth graphically and in the form of simplified representationsof curved double helices. The Software, CURVATURE, can thusbe used to investigate possible roles of curvature in modulationof gene expression and for location of curved portions of DNA,which may play an important role in sequence-specific protein-DNAinteractions.