Regulation of Nucleosome Stacking and Chromatin Compaction by the Histone H4 N-Terminal Tail-H2A Acidic Patch Interaction

Nucleosomes play a dual role in regulating transcription dynamics

Transcription has a mechanical component, as the translocation of the transcription machinery or RNA polymerase (RNAP) on DNA or chromatin is dynamically coupled to the chromatin torsion. This posits chromatin mechanics as a possible regulator of eukaryotic transcription, however, the modes and mechanisms of this regulation are elusive. Here, we first take a statistical mechanics approach to model the torsional response of topology-constrained chromatin. Our model recapitulates the experimentally observed weaker torsional stiffness of chromatin compared to bare DNA and proposes structural transitions of nucleosomes into chirally distinct states as the driver of the contrasting torsional mechanics. Coupling chromatin mechanics with RNAP translocation in stochastic simulations, we reveal a complex interplay of DNA supercoiling and nucleosome dynamics in governing RNAP velocity. Nucleosomes play a dual role in controlling the transcription dynamics. The steric barrier aspect of nucleosomes in the gene body counteracts transcription via hindering RNAP motion, whereas the chiral transitions facilitate RNAP motion via driving a low restoring torque upon twisting the DNA. While nucleosomes with low dissociation rates are typically transcriptionally repressive, highly dynamic nucleosomes offer less of a steric barrier and enhance the transcription elongation dynamics of weakly transcribed genes via buffering DNA twist. We use the model to predict transcription-dependent levels of DNA supercoiling in segments of the budding yeast genome that are in accord with available experimental data. The model unveils a paradigm of DNA supercoiling-mediated interaction between genes and makes testable predictions that will guide experimental design.

Progeria-based vascular model identifies networks associated with cardiovascular aging and disease

Hutchinson-Gilford Progeria syndrome (HGPS) is a lethal premature aging disorder caused by a de novo heterozygous mutation that leads to the accumulation of a splicing isoform of Lamin A termed progerin. Progerin expression deregulates the organization of the nuclear lamina and the epigenetic landscape. Progerin has also been observed to accumulate at low levels during normal aging in cardiovascular cells of adults that do not carry genetic mutations linked with HGPS. Therefore, the molecular mechanisms that lead to vascular dysfunction in HGPS may also play a role in vascular aging-associated diseases, such as myocardial infarction and stroke. Here, we show that HGPS patient-derived vascular smooth muscle cells (VSMCs) recapitulate HGPS molecular hallmarks. Transcriptional profiling revealed cardiovascular disease remodeling and reactive oxidative stress response activation in HGPS VSMCs. Proteomic analyses identified abnormal acetylation programs in HGPS VSMC replication fork complexes, resulting in reduced H4K16 acetylation. Analysis of acetylation kinetics revealed both upregulation of K16 deacetylation and downregulation of K16 acetylation. This correlates with abnormal accumulation of error-prone nonhomologous end joining (NHEJ) repair proteins on newly replicated chromatin. The knockdown of the histone acetyltransferase MOF recapitulates preferential engagement of NHEJ repair activity in control VSMCs. Additionally, we find that primary donor-derived coronary artery vascular smooth muscle cells from aged individuals show similar defects to HGPS VSMCs, including loss of H4K16 acetylation. Altogether, we provide insight into the molecular mechanisms underlying vascular complications associated with HGPS patients and normative aging.

Multiscale modeling reveals the ion-mediated phase separation of nucleosome core particles

Due to the vast length scale inside the cell nucleus, multiscale models are required to understand chromatin folding, structure, and dynamics and how they regulate genomic activities such as DNA transcription, replication, and repair. We study the interactions and structure of condensed phases formed by the universal building block of chromatin, the nucleosome core particle (NCP), using bottom-up multiscale coarse-grained (CG) simulations with a model extracted from all-atom MD simulations. In the presence of the multivalent cations Mg(H2O)62+ or CoHex3+, we analyze the internal structures of the NCP aggregates and the contributions of histone tails and ions to the aggregation patterns. We then derive a "super" coarse-grained (SCG) NCP model to study the macroscopic scale phase separation of NCPs. The SCG simulations show the formation of NCP aggregates with Mg(H2O)62+ concentration-dependent densities and sizes. Variation of the CoHex3+ concentrations results in highly ordered lamellocolumnar and hexagonal columnar phases in agreement with experimental data. The results give detailed insights into nucleosome interactions and for understanding chromatin folding in the cell nucleus.

Liquid-liquid phase separation (LLPS) in DNA and chromatin systems from the perspective of colloid physical chemistry

DNA is a highly charged polyelectrolyte and is prone to associative phase separation driven by the presence of multivalent cations, charged surfactants, proteins, polymers and colloids. The process of DNA phase separation induced by positively charged species is often called DNA condensation. Generally, it refers to either intramolecular DNA compaction (coil-globule transition) or intermolecular DNA aggregation with macroscopic phase separation, but the formation of a DNA liquid crystalline system is also displayed. This has traditionally been described by polyelectrolyte theory and qualitative (Flory-Huggins-based) polymer theory approaches. DNA in the cell nucleus is packed into chromatin wound around the histone octamer (a protein complex comprising two copies each of the four histone proteins H2A, H2B, H3 and H4) to form nucleosomes separated by linker DNA. During the last decade, the phenomenon of the formation of biomolecular condensates (dynamic droplets) by liquid-liquid phase separation (LLPS) has emerged as a generally important mechanism for the formation of membraneless organelles from proteins, nucleic acids and their complexes. DNA and chromatin droplet formation through LLPS has recently received much attention by in vitro as well as in vivo studies that established the importance of this for compartmentalisation in the cell nucleus. Here, we review DNA and chromatin LLPS from a general colloid physical chemistry perspective. We start with a general discussion of colloidal phase separation in aqueous solutions and review the original (pre-LLPS era) work on DNA (macroscopic) phase separation for simpler systems with DNA in the presence of multivalent cations and well-defined surfactants and colloids. Following that, we discuss and illustrate the similarities of such macroscopic phase separation with the general behaviour of LLPS droplet formation by associative phase separation for DNA-protein systems, including chromatin; we also note cases of segregative association. The review ends with a discussion of chromatin LLPS in vivo and its physiological significance.

The molecular basis for cellular function of intrinsically disordered protein regions

Intrinsically disordered protein regions exist in a collection of dynamic interconverting conformations that lack a stable 3D structure. These regions are structurally heterogeneous, ubiquitous and found across all kingdoms of life. Despite the absence of a defined 3D structure, disordered regions are essential for cellular processes ranging from transcriptional control and cell signalling to subcellular organization. Through their conformational malleability and adaptability, disordered regions extend the repertoire of macromolecular interactions and are readily tunable by their structural and chemical context, making them ideal responders to regulatory cues. Recent work has led to major advances in understanding the link between protein sequence and conformational behaviour in disordered regions, yet the link between sequence and molecular function is less well defined. Here we consider the biochemical and biophysical foundations that underlie how and why disordered regions can engage in productive cellular functions, provide examples of emerging concepts and discuss how protein disorder contributes to intracellular information processing and regulation of cellular function.

The Effects of Histone H2B Ubiquitylations on the Nucleosome Structure and Internucleosomal Interactions

Eukaryotic gene compaction takes place at multiple levels to package DNA to chromatin and chromosomes. Two of the most fundamental levels of DNA packaging are at the nucleosome and dinucleosome stacks. The nucleosome is the basic gene-packing unit and is composed of DNA wrapped around a histone core. Nucleosomes stack with one another for further compaction of DNA. The first stacking step leads to dinucleosome formation, which is driven by internucleosomal interactions between various parts of two nucleosomes. Histone proteins are rich targets for post-translational modifications, some of which affect the structure of the nucleosome and the interactions between nucleosomes. These effects are often implicated in the regulation of various genomic transactions. In particular, histone H2B ubiquitylation has been associated with facilitated transcription and hexasome formation. Here, we employed semi-synthetically ubiquitylated histone H2B and single-molecule FRET to investigate the effects of H2B ubiquitylations at lysine 34 (H2BK34) and lysine 120 (H2BK120) on the structure of the nucleosome and the interactions between two nucleosomes. Our results suggest that H2BK34 ubiquitylation widens the DNA gyre gap in the nucleosome and stabilizes long- and short-range internucleosomal interactions while H2BK120 ubiquitylation does not affect the nucleosome structure or internucleosomal interactions. These results suggest potential roles for H2B ubiquitylations in facilitated transcription and hexasome formation while maintaining the structural integrity of chromatin.

Chromatin Liquid-Liquid Phase Separation (LLPS) Is Regulated by Ionic Conditions and Fiber Length

The dynamic regulation of the physical states of chromatin in the cell nucleus is crucial for maintaining cellular homeostasis. Chromatin can exist in solid- or liquid-like forms depending on the surrounding ions, binding proteins, post-translational modifications and many other factors. Several recent studies suggested that chromatin undergoes liquid-liquid phase separation (LLPS) in vitro and also in vivo; yet, controversial conclusions about the nature of chromatin LLPS were also observed from the in vitro studies. These inconsistencies are partially due to deviations in the in vitro buffer conditions that induce the condensation/aggregation of chromatin as well as to differences in chromatin (nucleosome array) constructs used in the studies. In this work, we present a detailed characterization of the effects of K⁺, Mg²⁺ and nucleosome fiber length on the physical state and property of reconstituted nucleosome arrays. LLPS was generally observed for shorter nucleosome arrays (15-197-601, reconstituted from 15 repeats of the Widom 601 DNA with 197 bp nucleosome repeat length) at physiological ion concentrations. In contrast, gel- or solid-like condensates were detected for the considerably longer 62-202-601 and lambda DNA (~48.5 kbp) nucleosome arrays under the same conditions. In addition, we demonstrated that the presence of reduced BSA and acetate buffer is not essential for the chromatin LLPS process. Overall, this study provides a comprehensive understanding of several factors regarding chromatin physical states and sheds light on the mechanism and biological relevance of chromatin phase separation in vivo.

DNP-enhanced solid-state NMR spectroscopy of chromatin polymers

Chromatin is a DNA-protein polymer that represents the functional form of the genome. The main building block of chromatin is the nucleosome, a structure that contains 147 base pairs of DNA and two copies each of the histone proteins H2A, H2B, H3 and H4. Previous work has shown that magic angle spinning (MAS) NMR spectroscopy can capture the nucleosome at high resolution although studies have been challenging due to low sensitivity, the presence of dynamic and rigid components, and the complex interaction networks of nucleosomes within the chromatin polymer. Here, we use dynamic nuclear polarization (DNP) to enhance the sensitivity of MAS NMR experiments of nucleosome arrays at 100 K and show that well-resolved ¹³C-¹³C MAS NMR correlations can be obtained much more efficiently. We evaluate the effect of temperature on the chemical shifts and linewidths in the spectra and demonstrate that changes are relatively minimal and clustered in regions of histone-DNA or histone-histone contacts. We also compare samples prepared with and without DNA and show that the low temperature ¹³C-¹³C correlations exhibit sufficient resolution to detect chemical shift changes and line broadening for residues that form the DNA-histone interface. On the other hand, we show that the measurement of DNP-enhanced ¹⁵N-¹³C histone-histone interactions within the nucleosome core is complicated by the natural ¹³C abundance network in the sample. Nevertheless, the enhanced sensitivity afforded by DNP can be used to detect long-range correlations between histone residues and DNA. Overall, our experiments demonstrate that DNP-enhanced MAS NMR spectroscopy of chromatin samples yields spectra with high resolution and sensitivity and can be used to capture functionally relevant protein-DNA interactions that have implications for gene regulation and genome organization.

Targeting the Nucleosome Acidic Patch by Viral Proteins: Two Birds with One Stone?

The past decade illuminated the H2A-H2B acidic patch as a cornerstone for both nucleosome recognition and chromatin structure regulation. Higher-order folding of chromatin arrays is mediated by interactions of histone H4 tail with an adjacent nucleosome acidic patch. Dynamic chromatin folding ensures a proper regulation of nuclear functions fundamental to cellular homeostasis. Many cellular factors have been shown to act on chromatin by tethering nucleosomes via an arginine anchor binding to the acidic patch. This tethering mechanism has also been described for several viral proteins. In this minireview, we will discuss the structural basis for acidic patch engagement by viral proteins and the implications during respective viral infections. We will also discuss a model in which acidic patch occupancy by these non-self viral proteins alters the local chromatin state by preventing H4 tail-mediated higher-order chromatin folding.

Structural basis of chromatin regulation by histone variant H2A.Z

The importance of histone variant H2A.Z in transcription regulation has been well established, yet its mechanism-of-action remains enigmatic. Conflicting evidence exists in support of both an activating and a repressive role of H2A.Z in transcription. Here we report cryo-electron microscopy (cryo-EM) structures of nucleosomes and chromatin fibers containing H2A.Z and those containing canonical H2A. The structures show that H2A.Z incorporation results in substantial structural changes in both nucleosome and chromatin fiber. While H2A.Z increases the mobility of DNA terminus in nucleosomes, it simultaneously enables nucleosome arrays to form a more regular and condensed chromatin fiber. We also demonstrated that H2A.Z’s ability to enhance nucleosomal DNA mobility is largely attributed to its characteristic shorter C-terminus. Our study provides the structural basis for H2A.Z-mediated chromatin regulation, showing that the increase flexibility of the DNA termini in H2A.Z nucleosomes is central to its dual-functions in chromatin regulation and in transcription.

Structural features of nucleosomes in interphase and metaphase chromosomes

Structural heterogeneity of nucleosomes in functional chromosomes is unknown. Here, we devise the template-, reference- and selection-free (TRSF) cryo-EM pipeline to simultaneously reconstruct cryo-EM structures of protein complexes from interphase or metaphase chromosomes. The reconstructed interphase and metaphase nucleosome structures are on average indistinguishable from canonical nucleosome structures, despite DNA sequence heterogeneity, cell-cycle-specific posttranslational modifications, and interacting proteins. Nucleosome structures determined by a decoy-classifying method and structure variability analyses reveal the nucleosome structural variations in linker DNA, histone tails, and nucleosome core particle configurations, suggesting that the opening of linker DNA, which is correlated with H2A C-terminal tail positioning, is suppressed in chromosomes. High-resolution (3.4-3.5 Å) nucleosome structures indicate DNA-sequence-independent stabilization of superhelical locations ±0-1 and ±3.5-4.5. The linker histone H1.8 preferentially binds to metaphase chromatin, from which chromatosome cryo-EM structures with H1.8 at the on-dyad position are reconstituted. This study presents the structural characteristics of nucleosomes in chromosomes.

The solid and liquid states of chromatin

The review begins with a concise description of the principles of phase separation. This is followed by a comprehensive section on phase separation of chromatin, in which we recount the 60 years history of chromatin aggregation studies, discuss the evidence that chromatin aggregation intrinsically is a physiologically relevant liquid-solid phase separation (LSPS) process driven by chromatin self-interaction, and highlight the recent findings that under specific solution conditions chromatin can undergo liquid-liquid phase separation (LLPS) rather than LSPS. In the next section of the review, we discuss how certain chromatin-associated proteins undergo LLPS in vitro and in vivo. Some chromatin-binding proteins undergo LLPS in purified form in near-physiological ionic strength buffers while others will do so only in the presence of DNA, nucleosomes, or chromatin. The final section of the review evaluates the solid and liquid states of chromatin in the nucleus. While chromatin behaves as an immobile solid on the mesoscale, nucleosomes are mobile on the nanoscale. We discuss how this dual nature of chromatin, which fits well the concept of viscoelasticity, contributes to genome structure, emphasizing the dominant role of chromatin self-interaction.

Emerging Contributions of Solid-State NMR Spectroscopy to Chromatin Structural Biology

The eukaryotic genome is packaged into chromatin, a polymer of DNA and histone proteins that regulates gene expression and the spatial organization of nuclear content. The repetitive character of chromatin is diversified into rich layers of complexity that encompass DNA sequence, histone variants and post-translational modifications. Subtle molecular changes in these variables can often lead to global chromatin rearrangements that dictate entire gene programs with far reaching implications for development and disease. Decades of structural biology advances have revealed the complex relationship between chromatin structure, dynamics, interactions, and gene expression. Here, we focus on the emerging contributions of magic-angle spinning solid-state nuclear magnetic resonance spectroscopy (MAS NMR), a relative newcomer on the chromatin structural biology stage. Unique among structural biology techniques, MAS NMR is ideally suited to provide atomic level information regarding both the rigid and dynamic components of this complex and heterogenous biological polymer. In this review, we highlight the advantages MAS NMR can offer to chromatin structural biologists, discuss sample preparation strategies for structural analysis, summarize recent MAS NMR studies of chromatin structure and dynamics, and close by discussing how MAS NMR can be combined with state-of-the-art chemical biology tools to reconstitute and dissect complex chromatin environments.

Histone H4 lysine 20 mono-methylation directly facilitates chromatin openness and promotes transcription of housekeeping genes

Histone lysine methylations have primarily been linked to selective recruitment of reader or effector proteins that subsequently modify chromatin regions and mediate genome functions. Here, we describe a divergent role for histone H4 lysine 20 mono-methylation (H4K20me1) and demonstrate that it directly facilitates chromatin openness and accessibility by disrupting chromatin folding. Thus, accumulation of H4K20me1 demarcates highly accessible chromatin at genes, and this is maintained throughout the cell cycle. In vitro, H4K20me1-containing nucleosomal arrays with nucleosome repeat lengths (NRL) of 187 and 197 are less compact than unmethylated (H4K20me0) or trimethylated (H4K20me3) arrays. Concordantly, and in contrast to trimethylated and unmethylated tails, solid-state NMR data shows that H4K20 mono-methylation changes the H4 conformational state and leads to more dynamic histone H4-tails. Notably, the increased chromatin accessibility mediated by H4K20me1 facilitates gene expression, particularly of housekeeping genes. Altogether, we show how the methylation state of a single histone H4 residue operates as a focal point in chromatin structure control. While H4K20me1 directly promotes chromatin openness at highly transcribed genes, it also serves as a stepping-stone for H4K20me3-dependent chromatin compaction.

The effect of histone H4 lysine 20 methylation (H4K20me) on chromatin accessibility are not well established. Here the authors show how H4K20 methylation regulates chromatin structure and accessibility to ensure precise transcriptional outputs through the cell cycle using genome-wide approaches, in vitro biophysical assays, and NMR.

Soft-matter properties of multilayer chromosomes

This perspective aims to identify the relationships between the structural and dynamic properties of chromosomes and the fundamental properties of soft-matter systems. Chromatin is condensed into metaphase chromosomes during mitosis. The resulting structures are elongated cylinders having micrometer-scale dimensions. Our previous studies, using transmission electron microscopy, atomic force microscopy, and cryo-electron tomography, suggested that metaphase chromosomes have a multilayered structure, in which each individual layer has the width corresponding to a mononucleosome sheet. The self-assembly of multilayer chromatin plates from small chromatin fragments suggests that metaphase chromosomes are self-organized hydrogels (in which a single DNA molecule crosslinks the whole structure) with an internal liquid-crystal order produced by the stacking of chromatin layers along the chromosome axis. This organization of chromatin was unexpected, but the spontaneous assembly of large structures has been studied in different soft-matter systems and, according to these studies, the self-organization of chromosomes could be justified by the interplay between weak interactions of repetitive nucleosome building blocks and thermal fluctuations. The low energy of interaction between relatively large building blocks also justifies the easy deformation and structural fluctuations of soft-matter structures and the changes of phase caused by diverse external factors. Consistent with these properties of soft matter, different experimental results show that metaphase chromosomes are easily deformable. Furthermore, at the end of mitosis, condensed chromosomes undergo a phase transition into a more fluid structure, which can be correlated to the decrease in the Mg²⁺concentration and to the dissociation of condensins from chromosomes. Presumably, the unstacking of layers and chromatin fluctuations driven by thermal energy facilitate gene expression during interphase.

Histone tails cooperate to control the breathing of genomic nucleosomes

Genomic DNA is packaged in chromatin, a dynamic fiber variable in size and compaction. In chromatin, repeating nucleosome units wrap 145–147 DNA basepairs around histone proteins. Genetic and epigenetic regulation of genes relies on structural transitions in chromatin which are driven by intra- and inter-nucleosome dynamics and modulated by chemical modifications of the unstructured terminal tails of histones. Here we demonstrate how the interplay between histone H3 and H2A tails control ample nucleosome breathing motions. We monitored large openings of two genomic nucleosomes, and only moderate breathing of an engineered nucleosome in atomistic molecular simulations amounting to 24 μs. Transitions between open and closed nucleosome conformations were mediated by the displacement and changes in compaction of the two histone tails. These motions involved changes in the DNA interaction profiles of clusters of epigenetic regulatory aminoacids in the tails. Removing the histone tails resulted in a large increase of the amplitude of nucleosome breathing but did not change the sequence dependent pattern of the motions. Histone tail modulated nucleosome breathing is a key mechanism of chromatin dynamics with important implications for epigenetic regulation.

In the cell, the DNA is packed in chromatin. Chromatin is a highly dynamic fiber structure made of arrays of nucleosomes with different degrees of compaction. Each nucleosome has 145–147 basepairs of DNA wrapped around a protein octamer made of four unique histone proteins. Each histone is present twice and has a structured part and one or two disordered terminal tails. The regulation of gene expression in the cell and during cellular transitions depends on dynamic changes in chromatin structure. Chromatin dynamics are modulated by intra and inter nucleosome motions and by posttranslational chemical modifications of the histone tails. Here we reveal how histone tails control the intra nucleosome dynamics at atomic resolution. From extensive sampling of nucleosome dynamics in atomistic molecular simulations, we show that genomic nucleosomes breath more extensively than engineered ones and we describe how two histone tails cooperate to control nucleosome breathing through interactions between clusters of positively charged residues and the DNA. Nucleosome conformations with different degrees of opening are associated with different conformations, positions, and DNA interaction patterns of the tails. With this mechanism, we contribute to the understanding of chromatin dynamics at atomic resolution.

The dark side of histones: genomic organization and role of oncohistones in cancer

Background

The oncogenic role of histone mutations is one of the most relevant discovery in cancer epigenetics. Recurrent mutations targeting histone genes have been described in pediatric brain tumors, chondroblastoma, giant cell tumor of bone and other tumor types. The demonstration that mutant histones can be oncogenic and drive the tumorigenesis in pediatric tumors, led to the coining of the term “oncohistones.” The first identified histone mutations were localized at or near residues normally targeted by post-translational modifications (PTMs) in the histone N-terminal tails and suggested a possible interference with histone PTMs regulation and reading.

Main body

In this review, we describe the peculiar organization of the multiple genes that encode histone proteins, and the latter advances in both the identification and the biological role of histone mutations in cancer. Recent works show that recurrent somatic mutations target both N-terminal tails and globular histone fold domain in diverse tumor types. Oncohistones are often dominant-negative and occur at higher frequencies in tumors affecting children and adolescents. Notably, in many cases the mutations target selectively only some of the genes coding the same histone protein and are frequently associated with specific tumor types or, as documented for histone variant H3.3 in pediatric glioma, with peculiar tumors arising from specific anatomic locations.

Conclusion

The overview of the most recent advances suggests that the oncogenic potential of histone mutations can be exerted, together with the alteration of histone PTMs, through the destabilization of nucleosome and DNA–nucleosome interactions, as well as through the disruption of higher-order chromatin structure. However, further studies are necessary to fully elucidate the mechanism of action of oncohistones, as well as to evaluate their possible application to cancer classification, prognosis and to the identification of new therapies.

Histone tails as signaling antennas of chromatin

Histone tails, representing the N-terminal or C-terminal regions flanking the histone core, play essential roles in chromatin signaling networks. Intrinsic disorder of histone tails and their propensity for post-translational modifications allow them to serve as hubs in coordination of epigenetic processes within the nucleosomal context. Deposition of histone variants with distinct histone tail properties further enriches histone tails' repertoire in epigenetic signaling. Given the advances in experimental techniques and in silico modelling, we review the most recent data on histone tails' effects on nucleosome stability and dynamics, their function in regulating chromatin accessibility and folding. Finally, we discuss different molecular mechanisms to understand how histone tails are involved in nucleosome recognition by binding partners and formation of higher-order chromatin structures.

Histone Modifications in Papillomavirus Virion Minichromosomes

An unusual feature of papillomaviruses is that their genomes are packaged into virions along with host histones. Viral minichromosomes were visualized as “beads on a string” by electron microscopy in the 1970s but, to date, little is known about the posttranslational modifications of these histones. To investigate this, we analyzed the histone modifications in HPV16/18 quasivirions, wart-derived bovine papillomavirus (BPV1), and wart-derived human papillomavirus type 1 (HPV1) using quantitative mass spectrometry. The chromatin from all three virion samples had abundant posttranslational modifications (acetylation, methylation, and phosphorylation). These histone modifications were verified by acid urea polyacrylamide electrophoresis and immunoblot analysis. Compared to matched host cell controls, the virion minichromosome was enriched in histone modifications associated with active chromatin and depleted for those commonly found in repressed chromatin. We propose that the viral minichromosome acquires specific histone modifications late in infection that are coupled to the mechanisms of viral replication, late gene expression, and encapsidation. We predict that, in turn, these same modifications benefit early stages of infection by helping to evade detection, promoting localization of the viral chromosome to beneficial regions of the nucleus, and promoting early transcription and replication.

Beyond the Nucleosome: Nucleosome-Protein Interactions and Higher Order Chromatin Structure

The regulation of chromatin biology ultimately depends on the manipulation of its smallest subunit, the nucleosome. The proteins that bind and operate on the nucleosome do so, while their substrate is part of a polymer embedded in the dense nuclear environment. Their molecular interactions must in some way be tuned to deal with this complexity. Due to the rapid increase in the number of high-resolution structures of nucleosome-protein complexes and the increasing understanding of the cellular chromatin structure, it is starting to become clearer how chromatin factors operate in this complex environment. In this review, we analyze the current literature on the interplay between nucleosome-protein interactions and higher-order chromatin structure. We examine in what way nucleosomes-protein interactions can affect and can be affected by chromatin organization at the oligonucleosomal level. In addition, we review the characteristics of nucleosome-protein interactions that can cause phase separation of chromatin. Throughout, we hope to illustrate the exciting challenges in characterizing nucleosome-protein interactions beyond the nucleosome.

Linker histone defines structure and self-association behaviour of the 177 bp human chromatosome

Linker histones play essential roles in the regulation and maintenance of the dynamic chromatin structure of higher eukaryotes. The influence of human histone H1.0 on the nucleosome structure and biophysical properties of the resulting chromatosome were investigated and compared with the 177-bp nucleosome using Cryo-EM and SAXS. The 4.5 Å Cryo-EM chromatosome structure showed that the linker histone binds at the nucleosome dyad interacting with both linker DNA arms but in a tilted manner leaning towards one of the linker sides. The chromatosome is laterally compacted and rigid in the dyad and linker DNA area, in comparison with the nucleosome where linker DNA region is more flexible and displays structural variability. In solution, the chromatosomes appear slightly larger than the nucleosomes, with the volume increase compared to the bound linker histone, according to solution SAXS measurements. SAXS X-ray diffraction characterisation of Mg-precipitated samples showed that the different shapes of the 177 chromatosome enabled the formation of a highly ordered lamello-columnar phase when precipitated by Mg²⁺, indicating the influence of linker histone on the nucleosome stacking. The biological significance of linker histone, therefore, may be affected by the change in the polyelectrolyte and DNA conformation properties of the chromatosomes, in comparison to nucleosomes.

How potassium came to be the dominant biological cation: of metabolism, chemiosmosis, and cation selectivity since the beginnings of life

In the cytoplasm of practically all living cells, potassium is the major cation while sodium dominates in the media (seawater, extracellular fluids). Both prokaryotes and eukaryotes have elaborate mechanisms and spend significant energy to maintain this asymmetric K⁺ /Na⁺ distribution. This essay proposes an original line of evidence to explain how bacteria selected potassium at the very beginning of the evolutionary process and why it remains essential for eukaryotes.

Significant compaction of H4 histone tail upon charge neutralization by acetylation and its mimics, possible effects on chromatin structure

The intrinsically disordered, positively charged H4 histone tail is important for chromatin structure and function. We have explored conformational ensembles of human H4 tail in solution, with varying levels of charge neutralization via acetylation or amino-acid substitutions such as K→Q. We have employed an explicit water model shown recently to be well suited for simulations of intrinsically disordered proteins. Upon progressive neutralization of the H4, its radius of gyration decreases linearly with the tail charge q, the trend is explained using a simple polymer model. While the wild type state (q=+8) is essentially a random coil, hyper-acetylated H4 (q=+3) is virtually as compact and stable as a globular protein of the same number of amino-acids. Conformational ensembles of acetylated H4 match the corresponding K→X substitutions only approximately: based on the ensemble similarity, we propose K→M as a possible alternative to the commonly used K→Q. Possible effects of the H4 tail compaction on chromatin structure are discussed within a qualitative model in which the chromatin is highly heterogeneous, easily inter-converting between various structural forms. We predict that upon progressive charge neutralization of the H4 tail, the least compact sub-states of chromatin de-condense first, followed by de-condensation of more compact structures, e.g. those that harbor a high fraction of stacked di-nucleosomes. The predicted hierarchy of DNA accessibility increase upon progressive acetylation of H4 might be utilized by the cell for selective DNA accessibility control.

Weak interactions in higher-order chromatin organization

The detailed principles of the hierarchical folding of eukaryotic chromosomes have been revealed during the last two decades. Along with structures composing three-dimensional (3D) genome organization (chromatin compartments, topologically associating domains, chromatin loops, etc.), the molecular mechanisms that are involved in their establishment and maintenance have been characterized. Generally, protein-protein and protein-DNA interactions underlie the spatial genome organization in eukaryotes. However, it is becoming increasingly evident that weak interactions, which exist in biological systems, also contribute to the 3D genome. Here, we provide a snapshot of our current understanding of the role of the weak interactions in the establishment and maintenance of the 3D genome organization. We discuss how weak biological forces, such as entropic forces operating in crowded solutions, electrostatic interactions of the biomolecules, liquid-liquid phase separation, DNA supercoiling, and RNA environment participate in chromosome segregation into structural and functional units and drive intranuclear functional compartmentalization.

The human telomeric nucleosome displays distinct structural and dynamic properties

Telomeres protect the ends of our chromosomes and are key to maintaining genomic integrity during cell division and differentiation. However, our knowledge of telomeric chromatin and nucleosome structure at the molecular level is limited. Here, we aimed to define the structure, dynamics as well as properties in solution of the human telomeric nucleosome. We first determined the 2.2 Å crystal structure of a human telomeric nucleosome core particle (NCP) containing 145 bp DNA, which revealed the same helical path for the DNA as well as symmetric stretching in both halves of the NCP as that of the 145 bp ‘601’ NCP. In solution, the telomeric nucleosome exhibited a less stable and a markedly more dynamic structure compared to NCPs containing DNA positioning sequences. These observations provide molecular insights into how telomeric DNA forms nucleosomes and chromatin and advance our understanding of the unique biological role of telomeres.

Nucleosome allostery in pioneer transcription factor binding

While recent experiments revealed that some pioneer transcription factors (TFs) can bind to their target DNA sequences inside a nucleosome, the binding dynamics of their target recognitions are poorly understood. Here we used the latest coarse-grained models and molecular dynamics simulations to study the nucleosome-binding procedure of the two pioneer TFs, Sox2 and Oct4. In the simulations for a strongly positioning nucleosome, Sox2 selected its target DNA sequence only when the target was exposed. Otherwise, Sox2 entropically bound to the dyad region nonspecifically. In contrast, Oct4 plastically bound on the nucleosome mainly in two ways. First, the two POU domains of Oct4 separately bound to the two parallel gyres of the nucleosomal DNA, supporting the previous experimental results of the partial motif recognition. Second, the POU_S domain of Oct4 favored binding on the acidic patch of histones. Then, simulating the TFs binding to a genomic nucleosome, the LIN28B nucleosome, we found that the recognition of a pseudo motif by Sox2 induced the local DNA bending and shifted the population of the rotational position of the nucleosomal DNA. The redistributed DNA phase, in turn, changed the accessibility of a distant TF binding site, which consequently affected the binding probability of a second Sox2 or Oct4. These results revealed a nucleosomal DNA-mediated allosteric mechanism, through which one TF binding event can change the global conformation, and effectively regulate the binding of another TF at distant sites. Our simulations provide insights into the binding mechanism of single and multiple TFs on the nucleosome.

Specialized RSC: Substrate Specificities for a Conserved Chromatin Remodeler

The remodel the structure of chromatin (RSC) nucleosome remodeling complex is a conserved chromatin regulator with roles in chromatin organization, especially over nucleosome depleted regions therefore functioning in gene expression. Recent reports in Saccharomyces cerevisiae have identified specificities in RSC activity toward certain types of nucleosomes. RSC has now been shown to preferentially evict nucleosomes containing the histone variant H2A.Z in vitro. Furthermore, biochemical activities of distinct RSC complexes has been found to differ when their nucleosome substrate is partially unraveled. Mammalian BAF complexes, the homologs of yeast RSC and SWI/SNF complexes, are also linked to nucleosomes with H2A.Z, but this relationship may be complex and extent of conservation remains to be determined. The interplay of remodelers with specific nucleosome substrates and regulation of remodeler outcomes by nucleosome composition are tantalizing questions given the wave of structural data emerging for RSC and other SWI/SNF family remodelers.

Chromatin fibers stabilize nucleosomes under torsional stress

Torsional stress generated during DNA replication and transcription has been suggested to facilitate nucleosome unwrapping and thereby the progression of polymerases. However, the propagation of twist in condensed chromatin remains yet unresolved. Here, we measure how force and torque impact chromatin fibers with a nucleosome repeat length of 167 and 197. We find that both types of fibers fold into a left-handed superhelix that can be stabilized by positive torsion. We observe that the structural changes induced by twist were reversible, indicating that chromatin has a large degree of elasticity. Our direct measurements of torque confirmed the hypothesis of chromatin fibers as a twist buffer. Using a statistical mechanics-based torsional spring model, we extracted values of the chromatin twist modulus and the linking number per stacked nucleosome that were in good agreement with values measured here experimentally. Overall, our findings indicate that the supercoiling generated by DNA-processing enzymes, predicted by the twin-supercoiled domain model, can be largely accommodated by the higher-order structure of chromatin.

Intrinsic elasticity of nucleosomes is encoded by histone variants and calibrated by their binding partners

Histone variants fine-tune transcription, replication, DNA damage repair, and faithful chromosome segregation. Whether and how nucleosome variants encode unique mechanical properties to their cognate chromatin structures remains elusive. Here, using in silico and in vitro nanoindentation methods, extending to in vivo dissections, we report that histone variant nucleosomes are intrinsically more elastic than their canonical counterparts. Furthermore, binding proteins, which discriminate between histone variant nucleosomes, suppress this innate elasticity and also compact chromatin. Interestingly, when we overexpress the binding proteins in vivo, we also observe increased compaction of chromatin enriched for histone variant nucleosomes, correlating with diminished access. Taken together, these data suggest a plausible link between innate mechanical properties possessed by histone variant nucleosomes, the adaptability of chromatin states in vivo, and the epigenetic plasticity of the underlying locus.

Neutralization of the Positive Charges on Histone Tails by RNA Promotes an Open Chromatin Structure

RNA associates extensively with chromatin and can influence its structure; however, the potential role of the negative charges of RNA on chromatin structure remains unknown. Here, we demonstrate that RNA prevents precipitation of histones and can attenuate electrostatic interactions between histones and DNA, thereby loosening up the chromatin structure. This effect is independent of the sequence of RNA but dependent on its single-stranded nature, length, concentration, and negative charge. Opening and closure of chromatin by RNA occurs rapidly (within minutes) and passively (in permeabilized cells), in agreement with electrostatics. Accordingly, chromatin compaction following removal of RNA can be prevented by high ionic strength or neutralization of the positively charged histone tails by hyperacetylation. Finally, LINE1 repeat RNAs bind histone H2B and can decondense chromatin. We propose that RNA regulates chromatin opening and closure by neutralizing the positively charged tails of histones, reducing their electrostatic interactions with DNA.

Bridging chromatin structure and function over a range of experimental spatial and temporal scales by molecular modeling

Chromatin structure, dynamics, and function are being intensely investigated by a variety of methods, including microscopy, X-ray diffraction, nuclear magnetic resonance, biochemical crosslinking, chromosome conformation capture, and computation. A range of experimental techniques combined with modeling is clearly valuable to help interpret experimental data and, importantly, generate configurations and mechanisms related to the 3D organization and function of the genome. Contact maps, in particular, as obtained by a variety of chromosome conformation capture methods, are of increasing interest due to their implications on genome structure and regulation on many levels. In this perspective, using seven examples from our group's studies, we illustrate how molecular modeling can help interpret such experimental data. Specifically, we show how computed contact maps related to experimental systems can be used to explain structures of nucleosomes, chromatin higher-order folding, domain segregation mechanisms, gene organization, and the effect on chromatin structure of external and internal fiber parameters, such as nucleosome positioning, presence of nucleosome free regions, histone posttranslational modifications, and linker histone binding. We argue that such computations on multiple spatial and temporal scales will be increasingly important for the integration of genomic, epigenomic, and biophysical data on chromatin structure and related cellular processes.

Unraveling the multiplex folding of nucleosome chains in higher order chromatin

The DNA of eukaryotic chromatin and chromosomes is repeatedly supercoiled around histone octamers forming 'beads-on-a-string' chains of nucleosomes. The extent of nucleosome chain folding and DNA accessibility vary between different functional and epigenetic states of nuclear chromatin and change dramatically upon cell differentiation, but the molecular mechanisms that direct 3D folding of the nucleosome chain in vivo are still enigmatic. Recent advances in cell imaging and chromosome capture techniques have radically challenged the established paradigm of regular and hierarchical chromatin fibers by highlighting irregular chromatin organization and the importance of the nuclear skeletal structures hoisting the nucleosome chains. Here, we argue that, by analyzing individual structural elements of the nucleosome chain - nucleosome spacing, linker DNA conformations, internucleosomal interactions, and nucleosome chain flexibility - and integrating these elements in multiplex 3D structural models, we can predict the features of the multiplex chromatin folding assemblies underlying distinct developmental and epigenetic states in living cells. Furthermore, partial disassembly of the nuclear structures suspending chromatin fibers may reveal the intrinsic mechanisms of nucleosome chain folding. These mechanisms and structures are expected to provide molecular cues to modify chromatin structure and functions related to developmental and disease processes.

Post-translational modifications and chromatin dynamics

The dynamic structure of chromatin is linked to gene regulation and many other biological functions. Consequently, it is of importance to understand the factors that regulate chromatin dynamics. While the in vivo analysis of chromatin has verified that histone post-translational modifications play a role in modulating DNA accessibility, the complex nuclear environment and multiplicity of modifications prevents clear conclusions as to how individual modifications influence chromatin dynamics in the cell. For this reason, in vitro analyses of model reconstituted nucleosomal arrays has been pivotal in understanding the dynamic nature of chromatin compaction and the affects that specific post-translational modifications can have on the higher order chromatin structure. In this mini-review, we briefly describe the dynamic chromatin structures that have been observed in vitro and the environmental conditions that give rise to these various conformational states. Our focus then turns to a discussion of the specific histone post-translational modifications that have been shown to alter formation of these higher order chromatin structures in vitro and how this may relate to the biological state and accessibility of chromatin in vivo.

Mesoscale modeling reveals formation of an epigenetically driven HOXC gene hub

Gene expression is orchestrated at the structural level by nucleosome positioning, histone tail acetylation, and linker histone (LH) binding. Here, we integrate available data on nucleosome positioning, nucleosome-free regions (NFRs), acetylation islands, and LH binding sites to "fold" in silico the 55-kb HOXC gene cluster and investigate the role of each feature on the gene's folding. The gene cluster spontaneously forms a dynamic connection hub, characterized by hierarchical loops which accommodate multiple contacts simultaneously and decrease the average distance between promoters by ∼100 nm. Contact probability matrices exhibit "stripes" near promoter regions, a feature associated with transcriptional regulation. Interestingly, while LH proteins alone decrease long-range contacts and acetylation alone increases transient contacts, combined LH and acetylation produce long-range contacts. Thus, our work emphasizes how chromatin architecture is coordinated strongly by epigenetic factors and opens the way for nucleosome resolution models incorporating epigenetic modifications to understand and predict gene activity.

Rigid Basepair Monte Carlo Simulations of One-Start and Two-Start Chromatin Fiber Unfolding by Force

The organization of chromatin in 30 nm fibers remains a topic of debate. Here, we quantify the mechanical properties of the linker DNA and evaluate the impact of these properties on chromatin fiber folding. We extended a rigid basepair DNA model to include (un)wrapping of nucleosomal DNA and (un)stacking of nucleosomes in one-start and two-start chromatin fibers. Monte Carlo simulations that mimic single-molecule force spectroscopy experiments of folded nucleosomal arrays reveal different stages of unfolding as a function of force and are largely consistent with a two-start folding for 167 and one-start folding for 197 nucleosome repeat length fibers. The major insight is that nucleosome unstacking and subsequent unwrapping is not necessary to obtain quantitative agreement with experimental force extension curves up to the overstretching plateau of folded chromatin fibers at 3-5 pN. Nucleosome stacking appears better accommodated in one-start than in two-start conformations, and we suggest that this difference can compensate the increased energy for bending the linker DNA. Overall, these simulations capture the dynamic structure of chromatin fibers while maintaining realistic physical properties of the linker DNA.

Structure of an H1-Bound 6-Nucleosome Array Reveals an Untwisted Two-Start Chromatin Fiber Conformation

Chromatin adopts a diversity of regular and irregular fiber structures in vitro and in vivo. However, how an array of nucleosomes folds into and switches between different fiber conformations is poorly understood. We report the 9.7 Å resolution crystal structure of a 6-nucleosome array bound to linker histone H1 determined under ionic conditions that favor incomplete chromatin condensation. The structure reveals a flat two-start helix with uniform nucleosomal stacking interfaces and a nucleosome packing density that is only half that of a twisted 30-nm fiber. Hydroxyl radical footprinting indicates that H1 binds the array in an on-dyad configuration resembling that observed for mononucleosomes. Biophysical, cryo-EM, and crosslinking data validate the crystal structure and reveal that a minor change in ionic environment shifts the conformational landscape to a more compact, twisted form. These findings provide insights into the structural plasticity of chromatin and suggest a possible assembly pathway for a 30-nm fiber.

Revisit of Reconstituted 30-nm Nucleosome Arrays Reveals an Ensemble of Dynamic Structures

It has long been suggested that chromatin may form a fiber with a diameter of ~30 nm that suppresses transcription. Despite nearly four decades of study, the structural nature of the 30-nm chromatin fiber and conclusive evidence of its existence in vivo remain elusive. The key support for the existence of specific 30-nm chromatin fiber structures is based on the determination of the structures of reconstituted nucleosome arrays using X-ray crystallography and single-particle cryo-electron microscopy coupled with glutaraldehyde chemical cross-linking. Here we report the characterization of these nucleosome arrays in solution using analytical ultracentrifugation, NMR, and small-angle X-ray scattering. We found that the physical properties of these nucleosome arrays in solution are not consistent with formation of just a few discrete structures of nucleosome arrays. In addition, we obtained a crystal of the nucleosome in complex with the globular domain of linker histone H5 that shows a new form of nucleosome packing and suggests a plausible alternative compact conformation for nucleosome arrays. Taken together, our results challenge the key evidence for the existence of a limited number of structures of reconstituted nucleosome arrays in solution by revealing that the reconstituted nucleosome arrays are actually best described as an ensemble of various conformations with a zigzagged arrangement of nucleosomes. Our finding has implications for understanding the structure and function of chromatin in vivo.

Cryo-EM of nucleosome core particle interactions in trans

Nucleosomes, the basic unit of chromatin, are repetitively spaced along DNA and regulate genome expression and maintenance. The long linear chromatin molecule is extensively condensed to fit DNA inside the nucleus. How distant nucleosomes interact to build tertiary chromatin structure remains elusive. In this study, we used cryo-EM to structurally characterize different states of long range nucleosome core particle (NCP) interactions. Our structures show that NCP pairs can adopt multiple conformations, but, commonly, two NCPs are oriented with the histone octamers facing each other. In this conformation, the dyad of both nucleosome core particles is facing the same direction, however, the NCPs are laterally shifted and tilted. The histone octamer surface and histone tails in trans NCP pairs remain accessible to regulatory proteins. The overall conformational flexibility of the NCP pair suggests that chromatin tertiary structure is dynamic and allows access of various chromatin modifying machineries to nucleosomes.

Synergistic effects of H3 and H4 nucleosome tails on structure and dynamics of a lesion-containing DNA: Binding of a displaced lesion partner base to the H3 tail for GG-NER recognition

How DNA lesions in nucleosomes are recognized for global genome nucleotide excision repair (GG-NER) remains poorly understood, and the roles that histone tails may play remains to be established. Histone H3 and H4 N-terminal tails are of particular interest as their acetylation states are important in regulating nucleosomal functions in transcription, replication and repair. In particular the H3 tail has been the focus of recent attention as a site for the interaction with XPC, the GG-NER lesion recognition factor. Here we have investigated how the structure and dynamics of the DNA lesion cis-B[a]P-dG, derived from the environmental carcinogen benzo[a]pyrene (B[a]P), is impacted by the presence of flanking H3 and H4 tails. This lesion is well-repaired by GG-NER, and adopts a base-displaced/intercalated conformation in which the lesion partner C is displaced into the major groove. We used molecular dynamics simulations to obtain structural and dynamic characterizations for this lesion positioned in nucleosomal DNA so that it is bracketed by the H3 and H4 tails. The H4 tail was studied in unacetylated and acetylated states, while the H3 tail was unacetylated, its state when it binds XPC (Kakumu, Nakanishi et al., 2017). Our results reveal that upon acetylation, the H4 tail is released from the DNA surface; the H3 tail then forms a pocket that induces flipping and capture of the displaced lesion partner base C. This reveals synergistic effects of the behavior of the two tails. We hypothesize that the dual capability of the H3 tail to sense the displaced lesion partner base and to bind XPC could foster recognition of this lesion by XPC for initiation of GG-NER in nucleosomes.

Chromatin Fiber Folding Directed by Cooperative Histone Tail Acetylation and Linker Histone Binding

In eukaryotic chromatin, islands of histone tail acetylation are found near transcription start sites and enhancers, whereas linker histones (LHs) are localized in intergenic regions with wild-type (WT) histone tails. However, the structural mechanisms by which acetylation, in combination with LH binding, modulates chromatin compaction and hence transcription regulation are unknown. To explore the folding propensity by which these features may govern gene expression, we analyze 20 kb fibers that contain regularly spaced acetylation islands of two sizes (2 or 5 kb) with various LH levels by mesoscale modeling. Specifically, we investigate the effect of acetylating each histone tail (H3, H4, H2A, and H2B) individually, in combination (H3 and H4, or all tails), and adding LH to WT regions. We find that fibers with acetylated H4 tails lose local contacts (<1 kb) and fibers with all tails acetylated have decreased long-range contacts in those regions. Tail interaction plots show that this opening of the fiber is driven by the loss of tail-tail interactions in favor of tail-parent core interactions and/or increase in free tails. When adding LH to WT regions, the fibers undergo hierarchical looping, enriching long-range contacts between WT and acetylated domains. For reference, adding LH to the entire fiber results in local condensation and loss of overall long-range contacts. These findings highlight the cooperation between histone tail acetylation and regulatory proteins like LH in directing folding and structural heterogeneity of chromatin fibers. The results advance our understanding of chromatin contact domains, which represent a pivotal part of the cell cycle, diseased states, and differentiation states in eukaryotic cells.

Pathogens and Disease Play Havoc on the Host Epiproteome-The "First Line of Response" Role for Proteomic Changes Influenced by Disorder

Organisms face stress from multiple sources simultaneously and require mechanisms to respond to these scenarios if they are to survive in the long term. This overview focuses on a series of key points that illustrate how disorder and post-translational changes can combine to play a critical role in orchestrating the response of organisms to the stress of a changing environment. Increasingly, protein complexes are thought of as dynamic multi-component molecular machines able to adapt through compositional, conformational and/or post-translational modifications to control their largely metabolic outputs. These metabolites then feed into cellular physiological homeostasis or the production of secondary metabolites with novel anti-microbial properties. The control of adaptations to stress operates at multiple levels including the proteome and the dynamic nature of proteomic changes suggests a parallel with the equally dynamic epigenetic changes at the level of nucleic acids. Given their properties, I propose that some disordered protein platforms specifically enable organisms to sense and react rapidly as the first line of response to change. Using examples from the highly dynamic host-pathogen and host-stress response, I illustrate by example how disordered proteins are key to fulfilling the need for multiple levels of integration of response at different time scales to create robust control points.

A systematic analysis of nucleosome core particle and nucleosome-nucleosome stacking structure

Chromatin condensation is driven by the energetically favourable interaction between nucleosome core particles (NCPs). The close NCP-NCP contact, stacking, is a primary structural element of all condensed states of chromatin in vitro and in vivo. However, the molecular structure of stacked nucleosomes as well as the nature of the interactions involved in its formation have not yet been systematically studied. Here we undertake an investigation of both the structural and physico-chemical features of NCP structure and the NCP-NCP stacking. We introduce an “NCP-centred” set of parameters (NCP-NCP distance, shift, rise, tilt, and others) that allows numerical characterisation of the mutual positions of the NCPs in the stacking and in any other structures formed by the NCP. NCP stacking in more than 140 published NCP crystal structures were analysed. In addition, coarse grained (CG) MD simulations modelling NCP condensation was carried out. The CG model takes into account details of the nucleosome structure and adequately describes the long range electrostatic forces as well as excluded volume effects acting in chromatin. The CG simulations showed good agreement with experimental data and revealed the importance of the H2A and H4 N-terminal tail bridging and screening as well as tail-tail correlations in the stacked nucleosomes.

Probing Chromatin Structure with Magnetic Tweezers

Magnetic tweezers form a unique tool to study the topology and mechanical properties of chromatin fibers. Chromatin is a complex of DNA and proteins that folds the DNA in such a way that meter-long stretches of DNA fit into the micron-sized cell nucleus. Moreover, it regulates accessibility of the genome to the cellular replication, transcription, and repair machinery. However, the structure and mechanisms that govern chromatin folding remain poorly understood, despite recent spectacular improvements in high-resolution imaging techniques. Single-molecule force spectroscopy techniques can directly measure both the extension of individual chromatin fragments with nanometer accuracy and the forces involved in the (un)folding of single chromatin fibers. Here, we report detailed methods that allow one to successfully prepare in vitro reconstituted chromatin fibers for use in magnetic tweezers-based force spectroscopy. The higher-order structure of different chromatin fibers can be inferred from fitting a statistical mechanics model to the force-extension data. These methods for quantifying chromatin folding can be extended to study many other processes involving chromatin, such as the epigenetic regulation of transcription.

The 10-nm chromatin fiber and its relationship to interphase chromosome organization

A chromosome is a single long DNA molecule assembled along its length with nucleosomes and proteins. During interphase, a mammalian chromosome exists as a highly organized supramolecular globule in the nucleus. Here, we discuss new insights into how genomic DNA is packaged and organized within interphase chromosomes. Our emphasis is on the structural principles that underlie chromosome organization, with a particular focus on the intrinsic contributions of the 10-nm chromatin fiber, but not the regular 30-nm fiber. We hypothesize that the hierarchical globular organization of an interphase chromosome is fundamentally established by the self-interacting properties of a 10-nm zig-zag array of nucleosomes, while histone post-translational modifications, histone variants, and chromatin-associated proteins serve to mold generic chromatin domains into specific structural and functional entities.