Torsional stress can regulate the unwrapping of two outer half superhelical turns of nucleosomal DNA

DNA topology: A central dynamic coordinator in chromatin regulation

Double helical DNA winds around nucleosomes, forming a beads-on-a-string array that further contributes to the formation of high-order chromatin structures. The regulatory components of the chromatin, interacting intricately with DNA, often exploit the topological tension inherent in the DNA molecule. Recent findings shed light on, and simultaneously complicate, the multifaceted roles of DNA topology (also known as DNA supercoiling) in various aspects of chromatin regulation. Different studies may emphasize the dynamics of DNA topological tension across different scales, interacting with diverse chromatin factors such as nucleosomes, nucleic acid motors that propel DNA-tracking processes, and DNA topoisomerases. In this review, we consolidate recent studies and establish connections between distinct scientific discoveries, advancing our current understanding of chromatin regulation mediated by the supercoiling tension of the double helix. Additionally, we explore the implications of DNA topology and DNA topoisomerases in human diseases, along with their potential applications in therapeutic interventions.

From Nucleosomes to Compartments: Physicochemical Interactions Underlying Chromatin Organization

Chromatin organization plays a critical role in cellular function by regulating access to genetic information. However, understanding chromatin folding is challenging due to its complex, multiscale nature. Significant progress has been made in studying in vitro systems, uncovering the structure of individual nucleosomes and their arrays, and elucidating the role of physicochemical forces in stabilizing these structures. Additionally, remarkable advancements have been achieved in characterizing chromatin organization in vivo, particularly at the whole-chromosome level, revealing important features such as chromatin loops, topologically associating domains, and nuclear compartments. However, bridging the gap between in vitro and in vivo studies remains challenging. The resemblance between in vitro and in vivo chromatin conformations and the relevance of internucleosomal interactions for chromatin folding in vivo are subjects of debate. This article reviews experimental and computational studies conducted at various length scales, highlighting the significance of intrinsic interactions between nucleosomes and their roles in chromatin folding in vivo.

Structural and Dynamic Changes of Nucleosome upon GATA3 Binding

Pioneer factors, which can directly bind to nucleosomes, have been considered to change chromatin conformations. However, the binding impact on the nucleosome is little known. Here, we show how the pioneer factor GATA3 binds to nucleosomal DNA and affects the conformation and dynamics of nucleosomes by using a combination of SAXS, molecular modeling, and molecular dynamics simulations. Our structural models, consistent with the SAXS data, indicate that only one of the two DNA binding domains, N- and C-fingers, of GATA3 binds to an end of the DNA in solution. Our MD simulations further showed that the other unbound end of the DNA increases the fluctuation and enhances the DNA dissociation from the histone core when the N-finger binds to a DNA end, a site near the entry or exit of the nucleosome. However, this was not true for the binding of the C-finger that binds to a location about 15 base pairs distant from the DNA end. In this case, DNA dissociation occurred on the bound end. Taken together, we suggest that the N-finger and C-finger bindings of GATA3 commonly enhance DNA dissociation at one of the two DNA ends (the bound end for the C-finger binding and the unbound end for the N-finger binding), leading to triggering a conformational change in the chromatin.

Beyond gene expression: how MYC relieves transcription stress

MYC oncoproteins are key drivers of tumorigenesis. As transcription factors, MYC proteins regulate transcription by all three nuclear polymerases and gene expression. Accumulating evidence shows that MYC proteins are also crucial for enhancing the stress resilience of transcription. MYC proteins relieve torsional stress caused by active transcription, prevent collisions between the transcription and replication machineries, resolve R-loops, and repair DNA damage by participating in a range of protein complexes and forming multimeric structures at sites of genomic instability. We review the key complexes and multimerization properties of MYC proteins that allow them to mitigate transcription-associated DNA damage, and propose that the oncogenic functions of MYC extend beyond the modulation of gene expression.

Heterogeneous non-canonical nucleosomes predominate in yeast cells in situ

Nuclear processes depend on the organization of chromatin, whose basic units are cylinder-shaped complexes called nucleosomes. A subset of mammalian nucleosomes in situ (inside cells) resembles the canonical structure determined in vitro 25 years ago. Nucleosome structure in situ is otherwise poorly understood. Using cryo-electron tomography (cryo-ET) and 3D classification analysis of budding yeast cells, here we find that canonical nucleosomes account for less than 10% of total nucleosomes expected in situ. In a strain in which H2A-GFP is the sole source of histone H2A, class averages that resemble canonical nucleosomes both with and without GFP densities are found ex vivo (in nuclear lysates), but not in situ. These data suggest that the budding yeast intranuclear environment favors multiple non-canonical nucleosome conformations. Using the structural observations here and the results of previous genomics and biochemical studies, we propose a model in which the average budding yeast nucleosome's DNA is partially detached in situ.

Are extraordinary nucleosome structures more ordinary than we thought?

The nucleosome is a DNA-protein assembly that is the basic unit of chromatin. A nucleosome can adopt various structures. In the canonical nucleosome structure, 145-147 bp of DNA is wrapped around a histone heterooctamer. The strong histone-DNA interactions cause the DNA to be inaccessible for nuclear processes such as transcription. Therefore, the canonical nucleosome structure has to be altered into different, non-canonical structures to increase DNA accessibility. While it is recognised that non-canonical structures do exist, these structures are not well understood. In this review, we discuss both the evidence for various non-canonical nucleosome structures in the nucleus and the factors that are believed to induce these structures. The wide range of non-canonical structures is likely to regulate the amount of accessible DNA, and thus have important nuclear functions.

Genome modeling: From chromatin fibers to genes

The intricacies of the 3D hierarchical organization of the genome have been approached by many creative modeling studies. The specific model/simulation technique combination defines and restricts the system and phenomena that can be investigated. We present the latest modeling developments and studies of the genome, involving models ranging from nucleosome systems and small polynucleosome arrays to chromatin fibers in the kb-range, chromosomes, and whole genomes, while emphasizing gene folding from first principles. Clever combinations allow the exploration of many interesting phenomena involved in gene regulation, such as nucleosome structure and dynamics, nucleosome-nucleosome stacking, polynucleosome array folding, protein regulation of chromatin architecture, mechanisms of gene folding, loop formation, compartmentalization, and structural transitions at the chromosome and genome levels. Gene-level modeling with full details on nucleosome positions, epigenetic factors, and protein binding, in particular, can in principle be scaled up to model chromosomes and cells to study fundamental biological regulation.

Single-Molecule Human Nucleosome Spontaneously Ruptures under the Stress of Compressive Force: A New Perspective on Gene Stability and Epigenetic Pathways

Force manipulation on the biological entities from living cells to protein molecules has revealed many mechanical details of cell biology from resolving folding and unfolding pathways to finding molecular interaction forces. A nucleosome is the basic repeating unit of chromatin where the histone octamer is wrapped by DNA, important for gene stability and regulation. How the inner side of the DNA gets accessed by other DNA binding molecules has been a puzzle that has been intensively studied and debated, important to epigenetics, gene stability, and regulations. Here we report our observation of spontaneous ruptures of human nucleosomes under pico-Newton (pN) compressive force. The amplitude of the compressive force, a squeezing rather than pulling force, involved in our experiment is tens of pN, which can be thermally available by biological force fluctuation at room temperature and under physiological conditions. This kind of structural rupture can loosen up the DNA around the histone, which in turn makes the DNA accessible to transcription and epigenetic modifications.

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.

Free Energy Landscape of H2A-H2B Displacement From Nucleosome

Nucleosome reconstitution plays an important role in many cellular functions. As an initial step, H2A-H2B dimer displacement, which is accompanied by disruption of many of the interactions within the nucleosome, should occur. To understand how H2A-H2B dimer displacement occurs, an adaptively biased molecular dynamics (ABMD) simulation was carried out to generate a variety of displacements of the H2A-H2B dimer from the fully wrapped to partially unwrapped nucleosome structures. With regards to these structures, the free energy landscape of the dimer displacement was investigated using umbrella sampling simulations. We found that the main contributors to the free energy were the docking domain of H2A and the C-terminal of H4. There were various paths for the dimer displacement which were dependent on the extent of nucleosomal DNA wrapping, suggesting that modulation of the intra-nucleosomal interaction by external factors such as histone chaperons could control the path for the H2A-H2B dimer displacement. Key residues which contributed to the free energy have also been reported to be involved in the mutations and posttranslational modifications (PTMs) which are important for assembling and/or reassembling the nucleosome at the molecular level and are found in cancer cells at the phenotypic level. Our results give insight into how the H2A-H2B dimer displacement proceeds along various paths according to different interactions within the nucleosome.

DNA methylation cues in nucleosome geometry, stability and unwrapping

Cytosine methylation at the 5-carbon position is an essential DNA epigenetic mark in many eukaryotic organisms. Although countless structural and functional studies of cytosine methylation have been reported, our understanding of how it influences the nucleosome assembly, structure, and dynamics remains obscure. Here, we investigate the effects of cytosine methylation at CpG sites on nucleosome dynamics and stability. By applying long molecular dynamics simulations on several microsecond time scale, we generate extensive atomistic conformational ensembles of full nucleosomes. Our results reveal that methylation induces pronounced changes in geometry for both linker and nucleosomal DNA, leading to a more curved, under-twisted DNA, narrowing the adjacent minor grooves, and shifting the population equilibrium of sugar-phosphate backbone geometry. These DNA conformational changes are associated with a considerable enhancement of interactions between methylated DNA and the histone octamer, doubling the number of contacts at some key arginines. H2A and H3 tails play important roles in these interactions, especially for DNA methylated nucleosomes. This, in turn, prevents a spontaneous DNA unwrapping of 3-4 helical turns for the methylated nucleosome with truncated histone tails, otherwise observed in the unmethylated system on several microseconds time scale.

The kinetic landscape of nucleosome assembly: A coarse-grained molecular dynamics study

The organization of nucleosomes along the Eukaryotic genome is maintained over time despite disruptive events such as replication. During this complex process, histones and DNA can form a variety of non-canonical nucleosome conformations, but their precise molecular details and roles during nucleosome assembly remain unclear. In this study, employing coarse-grained molecular dynamics simulations and Markov state modeling, we characterized the complete kinetics of nucleosome assembly. On the nucleosome-positioning 601 DNA sequence, we observe a rich transition network among various canonical and non-canonical tetrasome, hexasome, and nucleosome conformations. A low salt environment makes nucleosomes stable, but the kinetic landscape becomes more rugged, so that the system is more likely to be trapped in off-pathway partially assembled intermediates. Finally, we find that the co-operativity between DNA bending and histone association enables positioning sequence motifs to direct the assembly process, with potential implications for the dynamic organization of nucleosomes on real genomic sequences.

Nucleosomes are biomolecular complexes formed by DNA wrapped around histone proteins. They represent the basic units of Eukaryotic chromosomes, compacting the genome so that it fits into the small nucleus, and regulating important biological processes such as gene expression. Nucleosomes are disassembled during disruptive events such as DNA replication, and re-assembled afterwards to preserve the correct organization of chromatin. However, the molecular details of nucleosome assembly are still not well understood. In particular, experiments found that histones and DNA may associate into a variety of non-canonical complexes, but their precise conformation and role during assembly remain unclear. In this study, we addressed these problems by performing extensive molecular dynamics simulations of nucleosomes undergoing assembly and disassembly. The simulations reveal many insights into the kinetics of assembly, the structure of non-canonical nucleosome intermediates, and the influence of salt concentration and DNA sequence on the assembly process.