30-nm Chromatin Fiber Research Articles

Structural basis for linker histone H5–nucleosome binding and chromatin fiber compaction

The hierarchical packaging of chromatin fibers plays a critical role in gene regulation. The 30-nm chromatin fibers, a central-level structure bridging nucleosomal arrays to higher-order organizations, function as the first level of transcriptional dormant chromatin. The dynamics of 30-nm chromatin fiber play a crucial role in biological processes related to DNA. Here, we report a 3.6-angstrom resolution cryogenic electron microscopy structure of H5-bound dodecanucleosome, i.e., the chromatin fiber reconstituted in the presence of linker histone H5, which shows a two-start left-handed double helical structure twisted by tetranucleosomal units. An atomic structural model of the H5-bound chromatin fiber, including an intact chromatosome, is built, which provides structural details of the full-length linker histone H5, including its N-terminal domain and an HMG-motif-like C-terminal domain. The chromatosome structure shows that H5 binds the nucleosome off-dyad through a three-contact mode in the chromatin fiber. More importantly, the H5-chromatin structure provides a fine molecular basis for the intra-tetranucleosomal and inter-tetranucleosomal interactions. In addition, we systematically validated the physiological functions and structural characteristics of the tetranucleosomal unit through a series of genetic and genomic studies in Saccharomyces cerevisiae and in vitro biophysical experiments. Furthermore, our structure reveals that multiple structural asymmetries of histone tails confer a polarity to the chromatin fiber. These findings provide structural and mechanistic insights into how a nucleosomal array folds into a higher-order chromatin fiber with a polarity in vitro and in vivo.

3D genome perspective on cell fate determination, organ regeneration, and diseases

The nucleosome is the fundamental subunit of chromatin. Nucleosome structures are formed by the combination of histone octamers and genomic DNA. Through a systematic and precise process of folding and compression, these structures form a 30-nm chromatin fibre that is further organized within the nucleus in a hierarchical manner, known as the 3D genome. Understanding the intricacies of chromatin structure and the regulatory mode governing chromatin interactions is essential for unravelling the complexities of cellular architecture and function, particularly in relation to cell fate determination, regeneration, and the development of diseases. Here, we provide a general overview of the hierarchical structure of chromatin as well as of the evolution of chromatin conformation capture techniques. We also discuss the dynamic regulatory changes in higher-order chromatin structure that occur during stem cell lineage differentiation and somatic cell reprogramming, potential regulatory insights at the chromatin level in organ regeneration, and aberrant chromatin regulation in diseases.

Chromatin unfolds and unwraps under tension and crowding

Genome is folded hierarchically in vivo. The genome structure is closely related to various functions, including transcription, gene regulation, and cell division. In near-atomistic level, DNA wraps around histones and forms “bead on a string” chromatin structure. Though in vitro study characterizes the stability of ordered two-start 30-nm chromatin fibers, in vivo study reveals the dominance of extended, disordered, and intertwined 10-nm arrays. To reconcile this controversy, we applied an accurate, near-atomistic coarse-grained model to study how chromatin reacts under tension and crowding effects, which are two important factors that mediate chromatin in living cells.

Simulation of Different Three-Dimensional Models of Whole Interphase Nuclei Compared to Experiments- A Consistent Scale-Bridging Simulation Framework for Genome Organization.

The three-dimensional architecture of chromosomes, their arrangement, and dynamics within cell nuclei are still subject of debate. Obviously, the function of genomes-the storage, replication, and transcription of genetic information-has closely coevolved with this architecture and its dynamics, and hence are closely connected. In this work a scale-bridging framework investigates how of the 30nm chromatin fibre organizes into chromosomes including their arrangement and morphology in the simulation of whole nuclei. Therefore, mainly two different topologies were simulated with corresponding parameter variations and comparing them to experiments: The Multi-Loop-Subcompartment (MLS) model, in which (stable) small loops form (stable) rosettes, connected by chromatin linkers, and the Random-Walk/Giant-Loop (RW/GL) model, in which large loops are attached to a flexible non-protein backbone, were simulated for various loop and linker sizes. The 30nm chromatin fibre was modelled as a polymer chain with stretching, bending and excluded volume interactions. A spherical boundary potential simulated the confinement to nuclei with different radii. Simulated annealing and Brownian Dynamics methods were applied in a four-step decondensation procedure to generate from metaphase decondensated interphase configurations at thermodynamical equilibrium. Both the MLS and the RW/GL models form chromosome territories, with different morphologies: The MLS rosettes result in distinct subchromosomal domains visible in electron and confocal laser scanning microscopic images. In contrast, the big RW/GL loops lead to a mostly homogeneous chromatin distribution. Even small changes of the model parameters induced significant rearrangements of the chromatin morphology. The low overlap of chromosomes, arms, and subchromosomal domains observed in experiments agrees only with the MLS model. The chromatin density distribution in CLSM image stacks reveals a bimodal behaviour in agreement with recent experiments. Combination of these results with a variety of (spatial distance) measurements favour an MLS like model with loops and linkers of 63 to 126kbp. The predicted large spaces between the chromatin fibres allow typically sized biological molecules to reach nearly every location in the nucleus by moderately obstructed diffusion and is in disagreement with the much simplified assumption that defined channels between territories for molecular transport as in the Interchromosomal Domain (ICD) hypothesis exist and are necessary for transport. All this is also in agreement with recent selective high-resolution chromosome interaction capture (T2C) experiments, the scaling behaviour of the DNA sequence, the dynamics of the chromatin fibre, the diffusion of molecules, and other measurements. Also all other chromosome topologies can in principle be excluded. In summary, polymer simulations of whole nuclei compared to experimental data not only clearly favour only a stable loop aggregate/rosette like genome architecture whose local topology is tightly connected to the global morphology and dynamics of the cell nucleus and hence can be used for understanding genome organization also in respect to diagnosis and treatment. This is in agreement with and also leads to a general novel framework of genome emergence, function, and evolution.

Interphase epichromatin: last refuge for the 30-nm chromatin fiber?

"Interphase epichromatin" describes the surface of chromatin located adjacent to the interphase nuclear envelope. It was discovered in 2011 using a bivalent anti-nucleosome antibody (mAb PL2-6), now known to be directed against the nucleosome acidic patch. The molecular structure of interphase epichromatin is unknown, but is thought to be heterochromatic with a high density of "exposed" acidic patches. In the 1960s, transmission electron microscopy of fixed, dehydrated, sectioned, and stained inactive chromatin revealed "unit threads," frequently organized into parallel arrays at the nuclear envelope, which were interpreted as regular helices with ~ 30-nm center-to-center distance. Also observed in certain cell types, the nuclear envelope forms a "sandwich" around a layer of closely packed unit threads (ELCS, envelope-limited chromatin sheets). Discovery of the nucleosome in 1974 led to revised helical models of chromatin. But these models became very controversial and the existence of in situ 30-nm chromatin fibers has been challenged. Development of cryo-electron microscopy (Cryo-EM) gave hope that in situ chromatin fibers, devoid of artifacts, could be structurally defined. Combining a contrast-enhancing phase plate and cryo-electron tomography (Cryo-ET), it is now possible to visualize chromatin in a "close-to-native" situation. ELCS are particularly interesting to study by Cryo-ET. The chromatin sheet appears to have two layers of ~ 30-nm chromatin fibers arranged in a criss-crossed pattern. The chromatin in ELCS is continuous with adjacent interphase epichromatin. It appears that hydrated ~ 30-nm chromatin fibers are quite rare in most cells, possibly confined to interphase epichromatin at the nuclear envelope.

The Dynamic Influence of Linker Histone Saturation within the Poly-Nucleosome Array

Linker histones bind to nucleosomes and modify chromatin structure and dynamics as a means of epigenetic regulation. Biophysical studies have shown that chromatin fibers can adopt a plethora of conformations with varying levels of compaction. Linker histone condensation, and its specific binding disposition, has been associated with directly tuning this ensemble of states. However, the atomistic dynamics and quantification of this mechanism remains poorly understood. Here, we present molecular dynamics simulations of octa-nucleosome arrays, based on a cryo-EM structure of the 30-nm chromatin fiber, with and without the globular domains of the H1 linker histone to determine how they influence fiber structures and dynamics. Results show that when bound, linker histones inhibit DNA flexibility and stabilize repeating tetra-nucleosomal units, giving rise to increased chromatin compaction. Furthermore, upon the removal of H1, there is a significant destabilization of this compact structure as the fiber adopts less strained and untwisted states. Interestingly, linker DNA sampling in the octa-nucleosome is exaggerated compared to its mono-nucleosome counterparts, suggesting that chromatin architecture plays a significant role in DNA strain even in the absence of linker histones. Moreover, H1-bound states are shown to have increased stiffness within tetra-nucleosomes, but not between them. This increased stiffness leads to stronger long-range correlations within the fiber, which may result in the propagation of epigenetic signals over longer spatial ranges. These simulations highlight the effects of linker histone binding on the internal dynamics and global structure of poly-nucleosome arrays, while providing physical insight into a mechanism of chromatin compaction.

Human metaphase chromosome consists of randomly arranged chromatin fibres with up to 30-nm diameter

During cell division, mitotic chromosomes assemble and are equally distributed into two new daughter cells. The chromosome organisation of the two chromatids is essential for even distribution of genetic materials. Although the 11-nm fibre or nucleosome structure is well-understood as a fundamental fibrous structure of chromosomes, the reports on organisation of 30-nm basic chromatin fibres have been controversial, with debates on the contribution of 30-nm or thicker fibres to the higher order inner structure of chromosomes. Here, we used focused ion beam/scanning electron microscopy (FIB/SEM) to show that both 11-nm and 30-nm fibres are present in the human metaphase chromosome, although the higher-order periodical structure could not be detected under the conditions employed. We directly dissected the chromosome every 10-nm and observed 224 cross-section SEM images. We demonstrated that the chromosome consisted of chromatin fibres of an average diameter of 16.9-nm. The majority of the chromatin fibres had diameters between 5 and 25-nm, while those with 30-nm were in the minority. The reduced packaging ratio of the chromatin fibres was detected at axial regions of each chromatid. Our results provide a strong basis for further discussions on the chromosome higher-order structure.

Modelling and DNA topology of compact 2-start and 1-start chromatin fibres.

We have investigated the structure of the most compact 30-nm chromatin fibres by modelling those with 2-start or 1-start crossed-linker organisations. Using an iterative procedure we obtained possible structural solutions for fibres of the highest possible compaction permitted by physical constraints, including the helical repeat of linker DNA. We find that this procedure predicts a quantized nucleosome repeat length (NRL) and that only fibres with longer NRLs (≥197 bp) can more likely adopt the 1-start organisation. The transition from 2-start to 1-start fibres is consistent with reported differing binding modes of the linker histone. We also calculate that in 1-start fibres the DNA constrains more torsion (as writhe) than 2-start fibres with the same NRL and that the maximum constraint obtained is in accord with previous experimental results. We posit that the coiling of the fibre is driven by overtwisting of linker DNA which, in the most compact forms - for example, in echinoderm sperm and avian erythrocytes - could adopt a helical repeat of ∼10 bp/turn. We argue that in vivo the total twist of linker DNA could be modulated by interaction with other abundant chromatin-associated proteins and by epigenetic modifications of the C-terminal tail of linker histones.

Dynamic chromatin organization in the cell.

The organization and regulation of genomic DNA as nuclear chromatin is necessary for proper DNA function inside living eukaryotic cells. While this has been extensively explored, no true consensus is currently reached regarding the exact mechanism of chromatin organization. The traditional view has assumed that the DNA is packaged into a hierarchy of structures inside the nucleus based on the regular 30-nm chromatin fiber. This is currently being challenged by the fluid-like model of the chromatin which views the chromatin as a dynamic structure based on the irregular 10-nm fiber. In this review, we focus on the recent progress in chromatin structure elucidation highlighting the paradigm shift in chromatin folding mechanism from the classical textbook perspective of the regularly folded chromatin to the more dynamic fluid-like perspective.

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.

Abstract ID: 90 Accounting for radiation-induced indirect damage on DNA with the GEANT4 code

The use of Monte Carlo (MC) simulations remains a powerful tool to study biological effects induced by ionizing radiations on living beings. Several MC codes, with different level of complexity, are commonly used in research fields such as nanodosimetry, radiotherapy, radiation protection, and space radiation. This work performed an upgrade of an existing model developed by Bernal et al. [1] for radiobiological purposes, for accounting for the indirect DNA damage produced by ionizing particles. The Geant4-DNA simulation toolkit was used to simulate physical, pre-chemical and chemical stages of the early DNA damage induced by protons. Liquid water was used as the medium for simulations. Two phase-space files were generated, one containing energy deposition events inside the region of interest (ROI), and another one with the position of chemical species produced by water radiolysis from 0.1 ps up to 1 ns. The information contained in both files was superposed on a genetic material model with atomic-resolution,consisting of several copies of 30-nm chromatin fibers. The B-DNA configuration was used. This work focused on the indirect damage produced by the hydroxyl radical (OH.) at the deoxy-ribose sugar sites, normally trough hydrogen abstraction. Corresponding damage yields were determined. The approach followed to account for indirect damage in DNA was the same used by other radiobiological codes [2] , [3] . The critical parameter considered here was the reaction radius, which was calculated from the Smoluchowski’s diffusion equation. Single, double, and total strand break yields produced by direct, indirect, and mixed mechanisms are reported.

Binding of DNA-bending non-histone proteins destabilizes regular 30-nm chromatin structure.

Why most of the in vivo experiments do not find the 30-nm chromatin fiber, well studied in vitro, is a puzzle. Two basic physical inputs that are crucial for understanding the structure of the 30-nm fiber are the stiffness of the linker DNA and the relative orientations of the DNA entering/exiting nucleosomes. Based on these inputs we simulate chromatin structure and show that the presence of non-histone proteins, which bind and locally bend linker DNA, destroys any regular higher order structures (e.g., zig-zag). Accounting for the bending geometry of proteins like nhp6 and HMG-B, our theory predicts phase-diagram for the chromatin structure as a function of DNA-bending non-histone protein density and mean linker DNA length. For a wide range of linker lengths, we show that as we vary one parameter, that is, the fraction of bent linker region due to non-histone proteins, the steady-state structure will show a transition from zig-zag to an irregular structure—a structure that is reminiscent of what is observed in experiments recently. Our theory can explain the recent in vivo observation of irregular chromatin having co-existence of finite fraction of the next-neighbor (i + 2) and neighbor (i + 1) nucleosome interactions.

Structure and Epigenetic Regulation of Chromatin Fibers.

In eukaryotes, genomic DNA is hierarchically packaged by histones into chromatin on several levels to fit inside the nucleus. As a central-level structure between nucleosomal arrays and higher-order chromatin organizations, the 30-nm chromatin fiber and its dynamics play a crucial role in gene regulation. However, despite considerable efforts over the past three decades, the fundamental structure and its dynamic regulation of chromatin fibers still remain as a big challenge in molecular biology. Here, we mainly summarize the most recent progress in elucidating the structure of the 30-nm chromatin fiber in vitro and epigenetic regulation of chromatin fibers by chromatin factors, particularly histone variants. In addition, we also discuss recent studies in unraveling the three-dimensional organization of chromatin fibers in situ by genomic approaches and electron microscopy.

C3-O-02Structure of 30-nm Chromatin Fiber Revealed by Cryo-electron Microscopy

Understanding the structure of 30-nm chromatin fiber, typically regarded as the secondary structure of DNA, is one of the keys to illuminating the molecular mechanism of transcription and epigenetic regulation. How nucleosome arrays fold into chromatin fibers has been remained elusive for over three decades. Cryo-electron microscopy (cryo-EM) provides a suitable tool for solving this puzzle. We have recently solved the 3-D cryo-EM structure of the 30-nm chromatin fiber reconstituted in vitro in the presence of linker histone H1 at resolution of about 11 Ä, which shows a histone H1-dependent left-handed double-helical architecture with twist of repeating tetra-nucleosomal structural units (Fig. 1 ) [ 1 ]. This result and some further EM studies on chromatin fibers will be presented. ...

Nucleosome Repeat Length Relates to the Gene Expression Level in Yeast

We are investigating correlation between the DNA folding in 30-nm chromatin fiber and the level of gene expression. The 30-nm fiber is generally characterized by the nucleosome repeat length (NRL) - that is, the length of the core DNA, 147 bp, plus the linker DNA length, L. We found previously that there are two families of the two-start chromatin fiber structures characterized by different DNA topology and flexibility. (Depending on the NRL value, the energetically optimal fiber structure belongs to one of the two families.) Here we analyze the high resolution nucleosome positioning data to find whether there is any correlation between the NRL and the gene expression level in yeast. We calculate the NRL values for the two groups of genes - 25% highly expressed and 25% lowly expressed genes (out of ∼3,500 yeast genes that are at least 1,000 bp long). Our results show that the average NRL=161-162 bp for the highly active genes (i.e., linker L=14-15 bp), whereas NRL=167-168 bp (i.e., linker L= 20-21 bp) for the lowly transcribed genes. Based on these findings, we conclude that the highly and lowly active gene sets have distinct nucleosome fiber organization with the linker L≈10n+5 and 10n, respectively. We hypothesize that organization of the most active genes in fibers with L≈10n+5 (which are more flexible than the fibers with L≈10n) facilitates formation of gene loops, thereby inducing transcription of these genes.

Chromatin structure revealed by X-ray scattering analysis and computational modeling

It remains unclear how the 2m of human genomic DNA is organized in each cell. The textbook model has long assumed that the 11-nm-diameter nucleosome fiber (beads-on-a-string), in which DNA is wrapped around core histones, is folded into a 30-nm chromatin fiber. One of the classical models assumes that the 30-nm chromatin fiber is further folded helically to form a larger fiber. Small-angle X-ray scattering (SAXS) is a powerful method for investigating the bulk structure of interphase chromatin and mitotic chromosomes. SAXS can detect periodic structures in biological materials in solution. In our SAXS results, no structural feature larger than 11nm was detected. Combining this with a computational analysis of “in silico condensed chromatin” made it possible to understand more about the X-ray scattering profiles and suggested that the chromatin in interphase nuclei and mitotic chromosomes essentially consists of irregularly folded nucleosome fibers lacking the 30-nm chromatin structure. In this article, we describe the experimental details of our SAXS and modeling systems. We also discuss other methods for investigating the chromatin structure in cells.

Chromosome Information Service (CIS), XXV

Since Flemming described a nuclear substance in the nineteenth century and named it “chromatin,” this substance has fascinated biologists. What is the structure of chromatin? DNA is wrapped around core histones, forming a nucleosome fiber (10-nm fiber). This fiber has long been assumed to fold into a 30-nm chromatin fiber and subsequently into helically folded larger fibers or radial loops. However, several recent studies, including our cryo-EM and X-ray scattering analyses, demonstrated that chromatin is composed of irregularly folded 10-nm fibers, without 30-nm chromatin fibers, in interphase chromatin and mitotic chromosomes. This irregular folding implies a chromatin state that is physically less constrained, which could be more dynamic compared with classical regular helical folding structures. Consistent with this, recently, the authors uncovered by single nucleosome imaging large nucleosome fluctuations in living mammalian cells (∼50 nm/30 ms). Subsequent computational modeling suggested that nucleosome fluctuation increases chromatin accessibility, which is advantageous for many “target searching” biological processes such as transcriptional regulation. Therefore, this review provides a novel view on chromatin structure in which chromatin consists of dynamic and disordered 10-nm fibers. 2.Brown EJ, Bachtrog D (2014) The chromatin landscape of Drosophila: Comparisons between species, sexes and chromosomes Genome Res 24 1125–1137.

ELCS in Ice: Cryo-electron Microscopy of Nuclear Envelope-Limited Chromatin Sheets

Nuclear envelope-limited chromatin sheets (ELCS) form during excessive interphase nuclear envelope growth in a variety of cells. ELCS appear as extended sheets within the cytoplasm connecting distant nuclear lobes. Cross-section stained images of ELCS, viewed by transmission electron microscopy, resemble a sandwich of apposed nuclear envelopes separated by ∼30 nm, containing a layer of parallel chromatin fibers. In this study, the ultrastructure of ELCS was compared by three different methods: (1) aldehyde fixation/dehydration/plastic embedding/sectioning and staining, (2) high-pressure freezing/freeze substitution into plastic/sectioning and staining, and (3) high-pressure freezing/cryo-sectioning/cryo-electron microscopy. ELCS could be clearly visualized by all three methods and, consequently, must exist in vivo and are not fixation artifacts. The ∼30-nm chromatin fibers could only be observed following aldehyde fixation; none were seen in cryo-sections. Electron microscopic tomography tangential views of aldehyde-fixed ELCS suggested an ordering of the separate chromatin fibers adjacent to the nuclear envelope. Possible mechanisms of this chromatin ordering are discussed.

New insights into the helical structure of 30-nm chromatin fibers.

In eukaryotic cells, genomic DNA is highly packaged into chromatin to fit inside the nucleus. The accessibility of DNA is dependent on the packing density of chromatin fibers, which plays a critical role in transcriptional regulation and all other DNA-related biological processes. Understanding the structure of chromatin is key to illuminating the functions and molecular mechanisms of chromatin dynamics in epigenetic regulation. The structure of nucleosome—the basic unit of chromatin—has been defined by crystal X-ray studies at high resolution; it consists of 147 base pairs (bp) of DNA wrapping around an octamer of histones (two copies each of H2A, H2B, H3 and H4) approximately 1.7 times (Luger et al., 1997). Despite considerable efforts during the last three decades, however, how the nucleosomes interact with each other in a “beads-on-a-string” nucleosomal array to form a condensed 30-nm chromatin fiber—typically regarded as the secondary structure of chromatin—still remains controversial (Robinson and Rhodes, 2006). Based on the studies of native 30-nm fibers in nuclei or isolated from nuclei by electron microscopy, small angle X-ray/neutron scattering, optical dichroism and analytical centrifugation (Finch and Klug, 1976; Gerchman and Ramakrishnan, 1987; Ghirlando and Felsenfeld, 2008; Langmore and Paulson, 1983; Widom and Klug, 1985), two basic classes of structural models had been proposed in which nucleosomes are either arranged linearly in a one-start solenoid-type helix with bent linker DNA or zigzag back and forth in a two-start stack of nucleosomes connected by a relatively straight DNA linker (Widom and Klug, 1985; Williams et al., 1986; Woodcock et al., 1984). However, it is very difficult to resolve nucleosomes and linker DNA to trace the paths of nucleosomal arrays under conditions in nuclei or in isolated native chromatins in these early studies; in addition, the heterogeneous properties of nucleosomes in native chromatin with different DNA sequences/linker lengths and different histone compositions/modifications make it difficult to define the detailed structure of chromatin fibers. The problem has been partially addressed by reconstituting chromatin fibers in vitro on regular tandem repeats of unique nucleosome-positioning DNA sequences with purified histone proteins (Dorigo et al., 2004), which greatly improved the reproducibility and uniformity for structural analysis and allowed for a dissection of the contribution of individual factors, such as different NRLs, linker histones and histone variants/modifications. Using this system, the 3D cryo-EM structures of 30-nm chromatin fibers reconstituted in vitro from arrays of 12 nucleosomes in the presence of linker histone H1 have been recently determined at resolution of about 11 Å, which clearly show a histone H1-dependent left-handed twist of the repeating tetra-nucleosomal structural units (Song et al., 2014). The structures constitute the largest fragments of chromatin solved at this resolution which is high enough to allow clearly defining the spatial location of all individual nucleosomes and tracing the path of linker DNA, and provide new insights into the helical structure of 30-nm chromatin fibers.

Cryo-electron microscopy reveals high-resolution structures of 30-nm chromatin fibers in vitro.

Cryo-electron microscopy reveals high-resolution structures of 30-nm chromatin fibers in vitro.