DNA sequence and methylation prescribe the inside-out conformational dynamics and bending energetics of DNA minicircles

Depletion of CpG dinucleotides in bacterial genomes may represent an adaptation to high temperatures

Dinucleotide biases have been widely investigated in the genomes of eukaryotes and viruses, but not in bacteria. We assembled a dataset of bacterial genomes (>15 000), which are representative of the genetic diversity in the kingdom Eubacteria, and we analyzed dinucleotide biases in relation to different traits. We found that TpA dinucleotides are the most depleted and that CpG dinucleotides show the widest dispersion. The abundances of both dinucleotides vary with genomic G + C content and show a very strong phylogenetic signal. After accounting for G + C content and phylogenetic inertia, we analyzed different bacterial lifestyle traits. We found that temperature preferences associate with the abundance of CpG dinucleotides, with thermophiles/hyperthemophiles being particularly depleted. Conversely, the TpA dinucleotide displays a bias that only depends on genomic G + C composition. Using predictions of intrinsic cyclizability we also show that CpG depletion may associate with higher DNA bendability in both thermophiles/hyperthermophiles and mesophiles, and that the former are predicted to have significantly more flexible genomes than the latter. We suggest that higher bendability is advantageous at high temperatures because it facilitates DNA positive supercoiling and that, through modulation of DNA mechanical properties, local or global CpG depletion controls genome organization, most likely not only in bacteria.

Dependence of nucleosome mechanical stability on DNA mismatches

The organization of nucleosomes into chromatin and their accessibility are shaped by local DNA mechanics. Conversely, nucleosome positions shape genetic variations, which may originate from mismatches during replication and chemical modification of DNA. To investigate how DNA mismatches affect the mechanical stability and the exposure of nucleosomal DNA, we used an optical trap combined with single-molecule FRET and a single-molecule FRET cyclization assay. We found that a single base-pair C-C mismatch enhances DNA bendability and nucleosome mechanical stability for the 601-nucleosome positioning sequence. An increase in force required for DNA unwrapping from the histone core is observed for single base-pair C-C mismatches placed at three tested positions: at the inner turn, at the outer turn, or at the junction of the inner and outer turn of the nucleosome. The results support a model where nucleosomal DNA accessibility is reduced by mismatches, potentially explaining the preferred accumulation of single-nucleotide substitutions in the nucleosome core and serving as the source of genetic variation during evolution and cancer progression. Mechanical stability of an intact nucleosome, that is mismatch-free, is also dependent on the species as we find that yeast nucleosomes are mechanically less stable and more symmetrical in the outer turn unwrapping compared to Xenopus nucleosomes.

Employing bimodal representations to predict DNA bendability within a self-supervised pre-trained framework

The bendability of genomic DNA, which measures the DNA looping rate, is crucial for numerous biological processes of DNA. Recently, an advanced high-throughput technique known as 'loop-seq' has made it possible to measure the inherent cyclizability of DNA fragments. However, quantifying the bendability of large-scale DNA is costly, laborious, and time-consuming. To close the gap between rapidly evolving large language models and expanding genomic sequence information, and to elucidate the DNA bendability's impact on critical regulatory sequence motifs such as super-enhancers in the human genome, we introduce an innovative computational model, named MIXBend, to forecast the DNA bendability utilizing both nucleotide sequences and physicochemical properties. In MIXBend, a pre-trained language model DNABERT and convolutional neural network with attention mechanism are utilized to construct both sequence- and physicochemical-based extractors for the sophisticated refinement of DNA sequence representations. These bimodal DNA representations are then fed to a k-mer sequence-physicochemistry matching module to minimize the semantic gap between each modality. Lastly, a self-attention fusion layer is employed for the prediction of DNA bendability. In conclusion, the experimental results validate MIXBend's superior performance relative to other state-of-the-art methods. Additionally, MIXBend reveals both novel and known motifs from the yeast. Moreover, MIXBend discovers significant bendability fluctuations within super-enhancer regions and transcription factors binding sites in the human genome.

Bending Unwinds DNA

In biology, DNA is often tightly bent to small radii. Solely based on the groove asymmetry, a 30-year-old theoretical paper predicted that such bending should unwind DNA, but this effect has not been directly experimentally quantified so far. We developed a ligation-based assay with nicked DNA circles of variable length, thereby decoupling the twist-dependent ligation efficiency from the large bending strain which dominates conventional circularization assays. We demonstrate that tightly bent DNA indeed unwinds to over 11 base pairs/turn, exactly as predicted. Our discovery requires reassessing the molecular mechanisms and energetics of all processes where DNA is tightly bent or relaxed again, including DNA packaging, gene regulation and expression.

Electrostatic encoding of genome organization principles within single native nucleosomes

The eukaryotic genome, first packed into nucleosomes of about 150 bp around the histone core, is organized into euchromatin and heterochromatin, corresponding to the A and B compartments, respectively. Here, we asked if individual nucleosomes in vivo know where to go. That is, do mono-nucleosomes by themselves contain A/B compartment information, associated with transcription activity, in their biophysical properties? We purified native mono-nucleosomes to high monodispersity and used physiological concentrations of biological polyamines to determine their condensability. The chromosomal regions known to partition into A compartments have low condensability and vice versa. In silico chromatin polymer simulations using condensability as the only input showed that biophysical information needed to form compartments is all contained in single native nucleosomes and no other factors are needed. Condensability is also strongly anticorrelated with gene expression, and especially so near the promoter region and in a cell type dependent manner. Therefore, individual nucleosomes in the promoter know whether the gene is on or off, and that information is contained in their biophysical properties. Comparison with genetic and epigenetic features suggest that nucleosome condensability is a very meaningful axis onto which to project the high dimensional cellular chromatin state. Analysis of condensability using various condensing agents including those that are protein-based suggests that genome organization principle encoded into individual nucleosomes is electrostatic in nature. Polyamine depletion in mouse T cells, by either knocking out ornithine decarboxylase (ODC) or inhibiting ODC, results in hyperpolarized condensability, suggesting that when cells cannot rely on polyamines to translate biophysical properties of nucleosomes to control gene expression and 3D genome organization, they accentuate condensability contrast, which may explain dysfunction known to occur with polyamine deficiency.

The impact of the sequence-dependent physical properties of DNA on chromatin dynamics

The local mechanical properties of DNA depend on local sequence. Here we review recent genomic, structural, and computational efforts at deciphering the "mechanical code", i.e., the mapping between sequence and mechanics. We then discuss works that suggest how evolution has exploited the mechanical code to control the energetics of DNA-deforming biological processes such as nucleosome organization, transcription factor binding, DNA supercoiling, gene regulation, and 3D chromatin organization. As a whole, these recent works suggest that DNA sequence in diverse organisms can encode regulatory information governing diverse processes via the mechanical code.

Predicting chromosomal compartments directly from the nucleotide sequence with DNA-DDA

Three-dimensional (3D) genome architecture is characterized by multi-scale patterns and plays an essential role in gene regulation. Chromatin conformation capturing experiments have revealed many properties underlying 3D genome architecture, such as the compartmentalization of chromatin based on transcriptional states. However, they are complex, costly and time consuming, and therefore only a limited number of cell types have been examined using these techniques. Increasing effort is being directed towards deriving computational methods that can predict chromatin conformation and associated structures. Here we present DNA-delay differential analysis (DDA), a purely sequence-based method based on chaos theory to predict genome-wide A and B compartments. We show that DNA-DDA models derived from a 20 Mb sequence are sufficient to predict genome wide compartmentalization at the scale of 100 kb in four different cell types. Although this is a proof-of-concept study, our method shows promise in elucidating the mechanisms responsible for genome folding as well as modeling the impact of genetic variation on 3D genome architecture and the processes regulated thereby.

Molecular dynamics simulations for the study of chromatin biology

The organization of Eukaryotic DNA into chromatin has profound implications for the processing of genetic information. In the past years, molecular dynamics (MD) simulations proved to be a powerful tool to investigate the mechanistic basis of chromatin biology. We review recent all-atom and coarse-grained MD studies revealing how the structure and dynamics of chromatin underlie its biological functions. We describe the latest method developments; the structural fluctuations of nucleosomes and the various factors affecting them; the organization of chromatin fibers, with particular emphasis on its liquid-like character; the interactions and dynamics of transcription factors on chromatin; and how chromatin organization is modulated by molecular motors acting on DNA.

H2A-H2B Histone Dimer Plasticity and Its Functional Implications

The protein core of the nucleosome is composed of an H3-H4 histone tetramer and two H2A-H2B histone dimers. The tetramer organizes the central 60 DNA bp, while H2A-H2B dimers lock the flanking DNA segments. Being positioned at the sides of the nucleosome, H2A-H2B dimers stabilize the overall structure of the nucleosome and modulate its dynamics, such as DNA unwrapping, sliding, etc. Such modulation at the epigenetic level is achieved through post-translational modifications and the incorporation of histone variants. However, the detailed connection between the sequence of H2A-H2B histones and their structure, dynamics and implications for nucleosome functioning remains elusive. In this work, we present a detailed study of H2A-H2B dimer dynamics in the free form and in the context of nucleosomes via atomistic molecular dynamics simulations (based on X. laevis histones). We supplement simulation results by comparative analysis of information in the structural databases. Particularly, we describe a major dynamical mode corresponding to the bending movement of the longest H2A and H2B α-helices. This overall bending dynamics of the H2A-H2B dimer were found to be modulated by its interactions with DNA, H3-H4 tetramer, the presence of DNA twist-defects with nucleosomal DNA and the amino acid sequence of histones. Taken together, our results shed new light on the dynamical mechanisms of nucleosome functioning, such as nucleosome sliding, DNA-unwrapping and their epigenetic modulation.

DNA methylation: Precise modulation of chromatin structure and dynamics

DNA methylation plays a vital role in epigenetic regulation in both plants and animals, and typically occurs at the 5-carbon position of the cytosine pyrimidine ring within the CpG dinucleotide steps. Cytosine methylation can alter DNA's geometry, mechanical and physico-chemical properties - thus influencing the molecular signaling events vital for transcription, replication and chromatin remodeling. Despite the profound effect cytosine methylation can have on DNA, the underlying atomistic mechanisms remain enigmatic. Many studies so far have produced controversial findings on how cytosine methylation dictates DNA flexibility and accessibility, nucleosome stability and dynamics. Here, we review the most recent experimental and computational studies that provide precise characterization of structure and function of cytosine methylation and its versatile roles in modulating DNA mechanics, nucleosome and chromatin structure, stability and dynamics. Moreover, the review briefly discusses the relationship between DNA methylation and nucleosome positioning, and the crosstalk between DNA methylation and histone tail modifications.

Recent Advances in Investigating Functional Dynamics of Chromatin

Dynamics spanning the picosecond-minute time domain and the atomic-subcellular spatial window have been observed for chromatin in vitro and in vivo. The condensed organization of chromatin in eukaryotic cells prevents regulatory factors from accessing genomic DNA, which requires dynamic stabilization and destabilization of structure to initiate downstream DNA activities. Those processes are achieved through altering conformational and dynamic properties of nucleosomes and nucleosome–protein complexes, of which delineating the atomistic pictures is essential to understand the mechanisms of chromatin regulation. In this review, we summarize recent progress in determining chromatin dynamics and their modulations by a number of factors including post-translational modifications (PTMs), incorporation of histone variants, and binding of effector proteins. We focus on experimental observations obtained using high-resolution techniques, primarily including nuclear magnetic resonance (NMR) spectroscopy, Förster (or fluorescence) resonance energy transfer (FRET) microscopy, and molecular dynamics (MD) simulations, and discuss the elucidated dynamics in the context of functional response and relevance.

OUP accepted manuscript

DNA mechanical properties play a critical role in every aspect of DNA-dependent biological processes. Recently a high throughput assay named loop-seq has been developed to quantify the intrinsic bendability of a massive number of DNA fragments simultaneously. Using the loop-seq data, we develop a software tool, DNAcycP, based on a deep-learning approach for intrinsic DNA cyclizability prediction. We demonstrate DNAcycP predicts intrinsic DNA cyclizability with high fidelity compared to the experimental data. Using an independent dataset from in vitro selection for enrichment of loopable sequences, we further verified the predicted cyclizability score, termed C-score, can well distinguish DNA fragments with different loopability. We applied DNAcycP to multiple species and compared the C-scores with available high-resolution chemical nucleosome maps. Our analyses showed that both yeast and mouse genomes share a conserved feature of high DNA bendability spanning nucleosome dyads. Additionally, we extended our analysis to transcription factor binding sites and surprisingly found that the cyclizability is substantially elevated at CTCF binding sites in the mouse genome. We further demonstrate this distinct mechanical property is conserved across mammalian species and is inherent to CTCF binding DNA motif.

Predicting the configuration and energy of DNA in a nucleosome by coarse-grain modelling

We present a novel algorithm which uses a coarse-grained model and an energy minimisation procedure to predict the sequence-dependent DNA configuration in a nucleosome together with its energetic cost.