Molecular dynamics simulations of a nucleosome and free DNA

All atom molecular dynamics simulations (10ns) of a nucleosome and of its 146 basepairs of DNA free in solution have been conducted. DNA helical parameters (Roll, Tilt, Twist, Shift, Slide, Rise) were extracted from each trajectory to compare the conformation, effective force constants, persistence length measures, and fluctuations of nucleosomal DNA to free DNA. The conformation of DNA in the nucleosome, as determined by helical parameters, is found to be largely within the range of thermally accessible values obtained for free DNA. DNA is found to be less flexible on the nucleosome than when free in solution, however such measures are length scale dependent. A method for disassembling and reconstructing the conformation and dynamics of the nucleosome using Fourier analysis is presented. Long length variations in the conformation of nucleosomal DNA are identified other than those associated with helix repeat. These variations are required to create a proposed tetrasome conformation or to qualitatively reconstruct the 1.75 turns of the nucleosome's superhelix. Reconstruction of free DNA using selected long wavelength variations in conformation can produce either a left-handed or a right-handed superhelix. The long wavelength variations suggest 146 basepairs is a natural length of DNA to wrap around the histone core.     

Molecular Dynamics Simulations of a Nucleosome and Free DNA

Abstract

All atom molecular dynamics simulations (10ns) of a nucleosome and of its 146 basepairs of DNA free in solution have been conducted. DNA helical parameters (Roll, Tilt, Twist, Shift, Slide, Rise) were extracted from each trajectory to compare the conformation, effective force constants, persistence length measures, and fluctuations of nucleosomal DNA to free DNA. The conformation of DNA in the nucleosome, as determined by helical parameters, is found to be largely within the range of thermally accessible values obtained for free DNA. DNA is found to be less flexible on the nucleosome than when free in solution, however such measures are length scale dependent. A method for disassembling and reconstructing the conformation and dynamics of the nucleosome using Fourier analysis is presented. Long length variations in the conformation of nucleosomal DNA are identified other than those associated with helix repeat. These variations are required to create a proposed tetrasome conformation or to qualitatively reconstruct the 1.75 turns of the nucleosome's superhelix. Reconstruction of free DNA using selected long wavelength variations in conformation can produce either a left-handed or a right-handed superhelix. The long wavelength variations suggest 146 basepairs is a natural length of DNA to wrap around the histone core.     

Dissecting DNA-histone interactions in the nucleosome by molecular dynamics simulations of DNA unwrapping

The nucleosome complex of DNA wrapped around a histone protein octamer organizes the genome of eukaryotes and regulates the access of protein factors to the DNA. We performed molecular dynamics simulations of the nucleosome in explicit water to study the dynamics of its histone-DNA interactions. A high-resolution histone-DNA interaction map was derived that revealed a five-nucleotide periodicity, in which the two DNA strands of the double helix made alternating contacts. On the 100-ns timescale, the histone tails mostly maintained their initial positions relative to the DNA, and the spontaneous unwrapping of DNA was limited to 1–2 basepairs. In steered molecular dynamics simulations, external forces were applied to the linker DNA to investigate the unwrapping pathway of the nucleosomal DNA. In comparison with a nucleosome without the unstructured N-terminal histone tails, the following findings were obtained: 1), Two main barriers during unwrapping were identified at DNA position ±70 and ±45 basepairs relative to the central DNA basepair at the dyad axis. 2), DNA interactions of the histone H3 N-terminus and the histone H2A C-terminus opposed the initiation of unwrapping. 3), The N-terminal tails of H2A, H2B, and H4 counteracted the unwrapping process at later stages and were essential determinants of nucleosome dynamics. Our detailed analysis of DNA-histone interactions revealed molecular mechanisms for modulating access to nucleosomal DNA via conformational rearrangements of its structure.     

Theoretical study of structural transition in a nucleosome at low ionic strength

The theoretical analysis of nucleosome stability at low ionic strength has been performed on the basis of consideration of different contributions to the free energy of compact state of the nucleosome DNA terminal regions. The proposed model explains: the fact of low-salt structural change; the transition point (approximately 1.7 mM NaCl) and width (approximately 1 mM); the shift of the transition to the higher salt concentrations in the case of histones tails removal by trypsin. According to the model the increase of electrostatic repulsion between neighbouring turns of DNA superhelix is the main cause of the unwinding of nucleosomal DNA terminal regions in the course of low-salt structural change. The interactions between histone (H2A-H2B) dimer and (H3-H4)2 tetramer provide the compact state of the nucleosomal DNA terminal regions. The existence of electrostatic interactions of nucleosomal DNA terminal regions with tetramer was suggested. These interactions can provide the compact state of nucleosomal DNA at physiological ionic strength even in the absence of (H2A-H2B) dimer.     

AFM imaging and theoretical modeling studies of sequence-dependent nucleosome positioning

Telomeric chromatin has different features with respect to bulk chromatin, since nucleosomal repeat along the chain is unusually short. We studied the role of telomeric DNA sequences on nucleosomal spacing in a model system. Nucleosomal arrays, assembled on a 1500-bp-long human telomeric DNA and on a DNA fragment containing 8 copies of the 601 strong nucleosome positioning sequence, have been studied at the single molecule level, by atomic force microscopy imaging. Random nucleosome positioning was found in the case of human telomeric DNA. On the contrary, nucleosome positioning on 601 DNA is characterized by preferential positions of nucleosome dyad axis each 200 bp. The AFM-derived nucleosome organization is in satisfactory agreement with that predicted by theoretical modeling, based on sequence-dependent DNA curvature and flexibility. The reported results show that DNA sequence has a main role, not only in mononucleosome thermodynamic stability, but also in the organization of nucleosomal arrays.     

A theoretical model for the prediction of sequence-dependent nucleosome thermodynamic stability

A theoretical model for predicting nucleosome thermodynamic stability in terms of DNA sequence is advanced. The model is based on a statistical mechanical approach, which allows the calculation of the canonical ensemble free energy involved in the competitive nucleosome reconstitution. It is based on the hypothesis that nucleosome stability mainly depends on the bending and twisting elastic energy to transform the DNA intrinsic superstructure into the nucleosomal structure. The ensemble average free energy is calculated starting from the intrinsic curvature, obtained by integrating the dinucleotide step deviations from the canonical B-DNA and expressed in terms of a Fourier series, in the framework of first-order elasticity. The sequence-dependent DNA flexibility is evaluated from the differential double helix thermodynamic stability. A large number of free-energy experimental data, obtained in different laboratories by competitive nucleosome reconstitution assays, are successfully compared to the theoretical results. They support the hypothesis that the stacking energies are the major factor in DNA rigidity and could be a measure of DNA stiffness. A dual role of DNA intrinsic curvature and flexibility emerges in the determination of nucleosome stability. The difference between the experimental and theoretical (elastic) nucleosome-reconstitution free energy for the whole pool of investigated DNAs suggests a significant role for the curvature-dependent DNA hydration and counterion interactions, which appear to destabilize nucleosomes in highly curved DNAs. This model represents an attempt to clarify the main features of the nucleosome thermodynamic stability in terms of physical-chemical parameters and suggests that in molecular systems with a large degree of complexity, the average molecular properties dominate over the local features, as in a statistical ensemble.     

Unwrapping of nucleosomal DNA ends: a multiscale molecular dynamics study

To permit access to DNA-binding proteins involved in the control and expression of the genome, the nucleosome undergoes structural remodeling including unwrapping of nucleosomal DNA segments from the nucleosome core. Here we examine the mechanism of DNA dissociation from the nucleosome using microsecond timescale coarse-grained molecular dynamics simulations. The simulations exhibit short-lived, reversible DNA detachments from the nucleosome and long-lived DNA detachments not reversible on the timescale of the simulation. During the short-lived DNA detachments, 9 bp dissociate at one extremity of the nucleosome core and the H3 tail occupies the space freed by the detached DNA. The long-lived DNA detachments are characterized by structural rearrangements of the H3 tail including the formation of a turn-like structure at the base of the tail that sterically impedes the rewrapping of DNA on the nucleosome surface. Removal of the H3 tails causes the long-lived detachments to disappear. The physical consistency of the CG long-lived open state was verified by mapping a CG structure representative of this state back to atomic resolution and performing molecular dynamics as well as by comparing conformation-dependent free energies. Our results suggest that the H3 tail may stabilize the nucleosome in the open state during the initial stages of the nucleosome remodeling process.     

Dual role of DNA intrinsic curvature and flexibility in determining nucleosome stability

A statistical mechanistic approach to evaluate the sequence-dependent thermodynamic stability of nucleosomes is proposed. The model is based on the calculation of the DNA intrinsic curvature, obtained by integrating the nucleotide step deviations from the canonical B-DNA structure, and on the evaluation of the first order elastic distortion energy to reach the nucleosomal superstructure. Literature data on the free energy of nucleosome formation as obtained by competitive nucleosome reconstitution of a significant pool of different DNA sequences were compared with the theoretical results, and a satisfactorily good correlation was found. A striking result of the comparison is the emergence of two opposite roles of the DNA intrinsic curvature and flexibility in determining nucleosome stability. Finally, the obtained results suggest that the curvature-dependent DNA hydration should play a relevant role in the sequence-dependent nucleosome stability.     

Effects of DNA sequence and histone-histone interactions on nucleosome placement

Using competitive reconstitution, we have refined the parameters for the binding of histone octamers to artificial nucleosome-positioning sequences of the form: (A/T3nn(G/C)3nn. We find that the optimal period between flexible segments is approximately 10.1 base-pairs, supporting the view that the DNA on the nucleosome surface is overwound. The strongest requirement for flexible DNA is near the protein dyad. However, we see no indication of changes in DNA helical repeat in this region. Using a series of repetitive sequences, we confirm that neither all A/T-rich nor all G/C-rich regions are identical in promoting nucleosome formation. Surprisingly, A/T-rich segments containing the TpA step, subject to purine-purine clash in the minor groove, favor nucleosome formation over sequences lacking this step. Short tracts of adenine residues are found to position on the histone surface like other A/T-rich regions, in the manner predicted by the direction of their sequence-directed bends as determined by electrophoretic methods. Tracts containing five adenine residues are extremely aniostropic in their flexibility and are strongly detrimental to nucleosome formation when positioned for major groove compression. Longer adenine tracts are found to position near the ends of the nucleosomal DNA. However, other positions may be occupied by an A12 tract, with only a minor penalty in the free energy of nucleosome formation. Overall, reconstituted nucleosome positions are translationally degenerate, suggesting a weak dependence on DNA flexibility for nucleosome positioning. Dinucleosomal reconstitutions on tandem dimers of the 5 S RNA gene of Lytechinus variegatus demonstrate a weak phasing dependence for the interaction between nucleosomes. This interaction is maximal for the 202 base-pair repeat and suggests a co-operative mechanism for the formation of ordered nucleosomal arrays based on a combination of DNA flexibility and nucleosome-nucleosome interactions.     

A critical role for linker DNA in higher-order folding of chromatin fibers

Nucleosome-nucleosome interactions drive the folding of nucleosomal arrays into dense chromatin fibers. A better physical account of the folding of chromatin fibers is necessary to understand the role of chromatin in regulating DNA transactions. Here, we studied the unfolding pathway of regular chromatin fibers as a function of single base pair increments in linker length, using both rigid base-pair Monte Carlo simulations and single-molecule force spectroscopy. Both computational and experimental results reveal a periodic variation of the folding energies due to the limited flexibility of the linker DNA. We show that twist is more restrictive for nucleosome stacking than bend, and find the most stable stacking interactions for linker lengths of multiples of 10 bp. We analyzed nucleosomes stacking in both 1- and 2-start topologies and show that stacking preferences are determined by the length of the linker DNA. Moreover, we present evidence that the sequence of the linker DNA also modulates nucleosome stacking and that the effect of the deletion of the H4 tail depends on the linker length. Importantly, these results imply that nucleosome positioning in vivo not only affects the phasing of nucleosomes relative to DNA but also directs the higher-order structure of chromatin.     

Evaluation of Elastic Rod Models with Long Range Interactions for Predicting Nucleosome Stability

Abstract

The ability of a dinucleotide-step based elastic-rod model of DNA to predict nucleosome binding free energies is investigated using four available sets of elastic parameters. We compare the predicted free energies to experimental values derived from nucleosome reconstitution experiments for 84 DNA sequences. Elastic parameters (conformation and stiffnessess) obtained from MD simulations are shown to be the most reliable predictors, as compared to those obtained from analysis of base-pair step melting temperatures, or from analysis of x-ray structures. We have also studied the effect of varying the folded conformation of nucleosomal DNA by means of our Fourier filtering knock-out and knock-in procedure. This study confirmed the above ranking of elastic parameters, and helped to reveal problems inherent in models using only a local elastic energy function. Long-range interactions were added to the elastic-rod model in an effort to improve its predictive ability. For this purpose a Debye-Huckel energy term with a single, homogenous point charge per base- pair was introduced. This term contains only three parameters,—its weight relative to the elastic energy, the Debye screening length, and a minimum sequence distance for including pairwise interactions between charges. After optimization of these parameters, our Debye-Huckel term is attractive, and yields the same level of correlation with experiment (R = 0.75) as was achieved merely by varying the nucleosomal shape in the elastic-rod model. We suggest this result indicates a linker DNA—histone attraction or, possibly, entropic effects, that lead to a stabilization of a nucleosome away from the ends of DNA segments longer than 147 bp. Such effects are not accounted for by a localized elastic energy model.     

A 'loop recapture' mechanism for ACF-dependent nucleosome remodeling

The ATPase ISWI is the molecular motor of several nucleosome remodeling complexes including ACF. We analyzed the ACF-nucleosome interactions and determined the characteristics of ACF-dependent nucleosome remodeling. In contrast to ISWI, ACF interacts symmetrically with DNA entry sites of the nucleosome. Two-color fluorescence cross-correlation spectroscopy measurements show that ACF can bind four DNA duplexes simultaneously in a complex that contains two Acf1 and ISWI molecules. Using bead-bound nucleosomal substrates, nucleosome movement by mechanisms involving DNA twisting was excluded. Furthermore, an ACF-dependent local detachment of DNA from the nucleosome was demonstrated in a novel assay based on the preferred intercalation of ethidium bromide to free DNA. The findings suggest a loop recapture mechanism in which ACF introduces a DNA loop at the nucleosomal entry site that propagates over the histone octamer surface and leads to nucleosome repositioning.     

Structure-based Analysis of DNA Sequence Patterns Guiding Nucleosome Positioning in vitro

Abstract

Recent studies of genome-wide nucleosomal organization suggest that the DNA sequence is one of the major determinants of nucleosome positioning. Although the search for underlying patterns encoded in nucleosomal DNA has been going on for about 30 years, our knowledge of these patterns still remains limited. Based on our evaluations of DNA deformation energy, we developed new scoring functions to predict nucleosome positioning. There are three principal differences between our approach and earlier studies: (i) we assume that the length of nucleosomal DNA varies from 146 to 147 bp; (ii) we consider the anisotropic flexibility of pyrimidine-purine (YR) dimeric steps in the context of their neighbors (e.g., YYRR versus RYRY); (iii) we postulate that alternating AT-rich and GC-rich motifs reflect sequence-dependent interactions between histone arginines and DNA in the minor groove. Using these functions, we analyzed 20 nucleosome positions mapped in vitro at single nucleotide resolution (including clones 601, 603, 605, the pGUB plasmid, chicken β-globin and three 5S rDNA genes). We predicted 15 of the 20 positions with 1-bp precision, and two positions with 2-bp precision. The predicted position of the ‘601’ nucleosome (i.e., the optimum of the computed score) deviates from the experimentally determined unique position by no more than 1 bp—an accuracy exceeding that of earlier predictions.

Our analysis reveals a clear heterogeneity of the nucleosomal sequences which can be divided into two groups based on the positioning ‘rules’ they follow. The sequences of one group are enriched by highly deformable YR/YYRR motifs at the minor-groove bending sites SHL ±3.5 and ±5.5, which is similar to the α-satellite sequence used in most crystallized nucleosomes. Apparently, the positioning of these nucleosomes is determined by the interactions between histones H2A/H2B and the terminal parts of nucleosomal DNA. In the other group (that includes the ‘601’ clone) the same YR/YYRR motifs occur predominantly at the sites SHL ±1.5. The interaction between the H3/H4 tetramer and the central part of the nucleosomal DNA is likely to be responsible for the positioning of nucleosomes of this group, and the DNA trajectory in these nucleosomes may differ in detail from the published structures.

Thus, from the stereochemical perspective, the in vitro nucleosomes studied here follow either an X-ray-like pattern (with strong deformations in the terminal parts of nucleosomal DNA), or an alternative pattern (with the deformations occurring predominantly in the central part of the nucleosomal DNA). The results presented here may be useful for genome-wide classification of nucleosomes, linking together structural and thermodynamic characteristics of nucleosomes with the underlying DNA sequence patterns guiding their positions.     

Nucleosomes in solution exist as a mixture of twist-defect states

The 2.0 A crystal structure of a nucleosome core particle in complex with a bivalent pyrrole-imidazole polyamide reveals that this "clamp" effectively crossbraces the two gyres of the DNA superhelix, thereby stabilizing the nucleosome against dissociation. Using X-ray crystallography and footprinting techniques, we show that the clamp preferentially binds nucleosomes over free DNA, and that nucleosomal DNA exists as a mixture of multiple twist-defect intermediates in solution. The nucleosomes exist in one of two different conformations in various crystal structures that trap twist-defect intermediates, even on a strong positioning sequence. Evidence has been obtained supporting the existence of twist-defect states in nucleosomal DNA in solution that are similar to those obtained in crystal structures. Our results also substantiate the idea that twist diffusion may represent an important means of altering the accessibility of nucleosomal DNA both in the presence and in the absence of ATP-dependent chromatin-remodelling enzymes.