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Nucleosome

A nucleosome is the basic repeating subunit of in eukaryotic cells, consisting of approximately 147 base pairs of DNA wrapped in 1.65 left-handed superhelical turns around a central composed of two copies each of the core histones H2A, H2B, , and H4. This histone core has a diameter of about 11 , and the DNA-histone complex is connected by stretches of , typically 20–60 base pairs in length, with the often binding to stabilize the structure. In its extended form, the nucleosome array appears as "beads on a " under , representing the first level of DNA compaction within the . Nucleosomes play essential roles in and function beyond mere packaging, achieving an initial sevenfold linear compaction of DNA to fit the approximately 2 meters of human into a roughly 6 micrometers in . They regulate access to genetic information by modulating structure, influencing processes such as transcription, , repair, and recombination through dynamic assembly, disassembly, and repositioning. modifications, such as and , and variants of core histones further fine-tune nucleosome stability and function, enabling epigenetic control of . The nucleosome was first identified in 1974 by through biochemical reconstitution experiments that revealed its repeating nature in . The high-resolution of the nucleosome core particle, determined in 1997 at 2.8 Å resolution, confirmed the detailed architecture and histone-DNA interactions, providing a foundation for understanding dynamics. Since then, advances in have highlighted nucleosome plasticity, including breathing, sliding, and unwrapping, which are critical for cellular processes.

Structure

Core Particle Composition

The nucleosome serves as the fundamental subunit of chromatin, comprising approximately 147 base pairs of double-stranded DNA wrapped around a histone octamer formed by two copies each of the core histone proteins H2A, H2B, H3, and H4. This octamer represents the protein core of the nucleosome core particle, excluding the linker histone H1, which associates with the intervening DNA between nucleosomes. The core histones assemble into specific dimeric units: H2A-H2B heterodimers and H3-H4 heterodimers, with two H3-H4 dimers combining to form a central (H3-H4)<sub>2</sub> tetramer that is flanked on either side by an H2A-H2B dimer, yielding the overall 2:2:2:2 stoichiometry of the octamer. This arrangement provides a stable scaffold for DNA packaging, with the stoichiometry ensuring one octamer per core particle. The atomic-level organization of the nucleosome core particle was first elucidated through X-ray crystallography at 2.8 Å resolution, revealing the detailed assembly of the histone octamer and its interactions with DNA. A subsequent refinement at 1.9 Å resolution provided further insights into solvent-mediated interactions and histone positioning within the core. Recent cryo-EM studies from 2023 to 2025 have confirmed and refined these structural features, validating that the nucleosome core particle has a diameter of about 11 nm and height of 5.7 nm, with the central histone octamer scaffold measuring approximately 6.5 nm in diameter and 5.7 nm in height.

Histone Organization and DNA Wrapping

The histone fold motif, a conserved structural element in core histones, consists of three alpha-helices connected by short loops within the globular domains, facilitating heterodimerization between H2A-H2B and H3-H4 pairs and subsequent assembly into the octamer. This motif's handshake-like configuration enables stable dimer formation, with the alpha-helices providing the primary surfaces for intermolecular contacts during octamer construction. The histone octamer forms a wedge-shaped scaffold, with a central (H3-H4)<sub>2</sub> tetramer serving as the core around which two H2A-H2B dimers attach laterally. This architecture is stabilized by four key histone-histone interfaces: two between the H3-H4 dimers within the tetramer and two between the tetramer and each H2A-H2B dimer, primarily mediated by the histone fold domains and additional structured loops. The N-terminal tail domains of the histones protrude from this core, extending beyond the DNA wrapping region. Approximately 147 base pairs of DNA wrap around the octamer in 1.65-1.7 left-handed superhelical turns, forming the nucleosome core particle. This path positions the DNA backbone in close proximity to the histone surfaces, where it contacts roughly 89 residues across the octamer through predominantly electrostatic interactions between positively charged histone side chains and the negatively charged DNA phosphates. Stabilizing these contacts are arginine anchors, such as those from H3 and H4, which insert into the DNA minor groove at 10-14 sites along the superhelix, enhancing binding via shape readout and electrostatic attraction in narrowed groove regions. Complementary phosphate clamps, formed by histone loops (e.g., from H2B and H4), grip the DNA backbone at intervals, further securing the wrap and distributing bending strain. The DNA undergoes significant bending, averaging ~140° per turn around each pair, with local deformations driven by positive roll angles at minor groove-facing sites. The entry and exit points of the DNA are oriented approximately 80° apart relative to the octamer axis, reflecting the incomplete superhelical turn and facilitating formation without involvement here. Recent cryo-EM studies from 2024-2025 have uncovered subtle asymmetries in DNA-histone contacts, such as uneven distribution of insertions and interactions across the superhelix, which modulate local stability and influence nucleosome positioning preferences.

Histone Tails

Histone tails are flexible, unstructured extensions rich in positively charged and residues that protrude from the globular domains of the core in the nucleosome core particle (NCP). These tails, located at the N-termini of H2B, , and H4, and both N- and C-termini of H2A, lack a defined secondary structure and extend outward from the histone fold regions. For instance, the N-terminal tail comprises approximately the first 40 residues, while the H4 N-terminal tail spans the initial 20 residues, enabling their mobility within the nucleosomal context. The positions of these tails allow them to emerge from specific points between the two gyres of DNA wrapped around the , facilitating interactions both within the same NCP (intra-nucleosomal) and with neighboring nucleosomes (inter-nucleosomal). In the NCP, tails such as the N-tail can thread through the DNA superhelix, contacting multiple DNA segments, while others remain more solvent-exposed. These tails contribute to the of the NCP through weak, electrostatic interactions with nucleosomal DNA and adjacent histone surfaces, helping to compact and maintain the integrity of the core particle. A key example is the H4 N-terminal tail, which binds to the acidic patch on the H2A-H2B dimer within the octamer, reinforcing histone-DNA associations via charge complementarity. Similarly, the C-terminal tail of docks onto the surface of the H2B histone in the same nucleosome, stabilizing the dimer interface through hydrophobic and electrostatic contacts. The lengths and sequences of histone tails exhibit high evolutionary , particularly in motifs enriched with residues that underpin their DNA-binding and structural roles across eukaryotes. These conserved motifs, such as lysine-rich stretches in and H4 tails, have persisted due to their essential contributions to nucleosome architecture. Recent cryo-electron (cryo-EM) studies have revealed that histone tails adopt a range of conformations within the NCP, ranging from DNA-bound states to more extended forms, which modulate nucleosome compactness while retaining partial order rather than complete disorder. These dynamic yet structured poses influence the overall stability of the core particle without disrupting the canonical histone-DNA wrapping. Histone tails serve as primary sites for post-translational modifications, such as and , which can subtly alter their interactions within the nucleosome.

Higher-Order Chromatin Folding

Nucleosomes are connected by stretches of , typically ranging from 20 to 80 base pairs in length, which plays a crucial role in determining the spacing between adjacent nucleosomes and influencing the overall architecture. This variability in linker length, observed across species, tissues, and even within individual genomes, modulates the flexibility and compaction potential of the nucleosome chain. Shorter linkers promote tighter packing, while longer ones allow for more extended conformations, affecting higher-order folding. The assembly of nucleosomes into higher-order structures has been proposed to involve the formation of the 30-nm fiber, a compacted form of the 11-nm nucleosomal array, although its existence as a regular structure remains controversial, with recent studies (as of 2025) suggesting more dynamic, irregular configurations such as melts or liquid-like domains. Two primary models describe this fiber: the model, featuring a one-start helical stacking of nucleosomes with winding around the , and the model, characterized by a two-start where nucleosomes alternate sides with straight, crossed segments. The binds to the entry and exit points of DNA on the nucleosome, stabilizing the fiber by neutralizing charges and promoting internucleosomal interactions, which is essential for achieving the 30-nm diameter. This H1-mediated compaction results in approximately a 6- to 7-fold reduction in length compared to the extended nucleosomal array, representing a key step in DNA packaging. Further compaction beyond the 30-nm fiber occurs during processes like mitosis, leading to metaphase chromosomes with an overall DNA packing ratio exceeding 10,000-fold relative to linear DNA, though the precise contribution from the 30-nm stage to this final state varies with additional looping and scaffolding. Recent studies have highlighted how nucleosome spacing fine-tunes higher-order chromatin folding. For instance, linker lengths around 25-30 bp alter fiber regularity and phase separation propensity, with shorter linkers (e.g., 25 bp) inducing irregular orientations that enhance inter-fiber interactions and liquid-liquid phase separation at lower salt concentrations, while 30 bp linkers favor compact, stable intra-fiber stacking. Additionally, advances in 2024-2025 reveal that heterogeneous nanoscopic packing domains, on the scale of tens to hundreds of nanometers, emerge through the interplay of nucleosome remodeling and loop extrusion mechanisms, creating conformationally diverse regions that deviate from uniform fiber models. Beyond the 30-nm fiber, organizes into looped domains mediated by factors like and , which extrude loops to form topologically associating domains (TADs) that constrain interactions and enhance compartmentalization. Phase-separated condensates represent another higher-order state, where multivalent interactions drive the formation of liquid-like droplets that concentrate and regulatory proteins, further compacting and segregating genomic regions. These structures collectively enable dynamic, hierarchical folding essential for organization.

Assembly

In Vitro Methods

One of the foundational techniques for in vitro nucleosome reconstitution is the salt dialysis method, which involves mixing purified histone octamers with DNA in a high-salt buffer (typically 2 M NaCl) to disrupt electrostatic interactions, followed by gradual dialysis to lower salt concentrations to physiological levels (around 150 mM), allowing the histones to spontaneously wrap DNA into nucleosome core particles. This approach yields well-folded nucleosomes with approximately 147 base pairs of DNA wrapped around the histone octamer, mimicking the core structure observed in vivo, and has been widely used since the 1970s for biophysical and structural studies. To facilitate sequential deposition of histone components, in vitro methods often incorporate histone chaperones such as nucleosome assembly protein 1 (NAP-1), which binds H2A-H2B dimers and promotes their transfer onto pre-assembled (H3-H4)₂ tetramers bound to DNA, enhancing assembly efficiency under low-salt conditions. NAP-1-mediated assembly avoids non-specific aggregation and allows for controlled incorporation of histone variants, producing positioned nucleosomes suitable for downstream assays. For enhanced stability in structural investigations, disulfide-crosslinked nucleosome cores are generated by engineering cysteine mutations at strategic positions in histone proteins, such as between and H4 or H2A and H2B, followed by oxidation to form covalent bonds that lock the octamer in place after reconstitution via salt or chaperone-assisted methods. These stabilized particles resist disassembly during purification and enable high-resolution techniques like and cryo-electron microscopy (cryo-EM). Recent advancements (2023–2025) include the use of engineered cohesive-ended DNA fragments for assembling circular arrays, building on a 2021 method where short, complementary overhangs on nucleosomal DNA promote ligation into closed minicircles or arrays that maintain periodic nucleosome positioning without linker dependency. These constructs are particularly compatible with cryo-EM, as demonstrated in studies of variant nucleosomes and protein complexes, allowing visualization of dynamic conformations at near-atomic resolution while avoiding artifacts from linear arrays. Additionally, a 2024 method enables assembly of complete eukaryotic nucleosomes and (H3-H4)-only tetrasomes in by expressing core histones, mimicking stepwise eukaryotic deposition and facilitating production of non-canonical particles for structural studies. In vitro methods offer precise control over nucleosome positioning and composition for biophysical assays, such as single-molecule tracking of sliding dynamics, but they often exhibit non-physiological due to the absence of cellular regulatory factors. These techniques parallel assembly pathways in their stepwise deposition but lack the spatiotemporal regulation found in cells.

In Vivo Pathways

In vivo nucleosome assembly primarily occurs through replication-coupled pathways during , where parental histones are recycled and new histones are synthesized to maintain integrity behind the replication fork. This process ensures the duplication of nucleosomes in a manner coordinated with , preventing exposure of naked DNA and preserving epigenetic marks. The assembly begins with the deposition of an H3-H4 tetramer onto newly replicated DNA, facilitated by the chromatin assembly factor 1 (CAF-1) complex, which binds to (PCNA) at the replication fork. CAF-1 recognizes the replication-coupled context through this PCNA interaction, delivering the (H3-H4)<sub>2</sub> tetramer to form a tetrasome intermediate that wraps approximately 70 base pairs of DNA. Subsequently, two H2A-H2B dimers are added sequentially to complete the octameric nucleosome core particle, with chaperones like Nap1 or sNASP aiding in dimer placement. Beyond replication, nucleosomes assemble through replication-independent pathways during processes such as transcription elongation or , where different histone chaperones mediate histone deposition to restore structure. For instance, during transcription, the FACT complex recycles and reassembles nucleosomes to facilitate passage, while in double-strand break repair, the HIRA chaperone deposits H3.3-containing nucleosomes to fill gaps. Recent structural studies from 2024 have elucidated the molecular details of CAF-1's histone binding, revealing how its disordered regions and folded modules interact with H3-H4 dimers to ensure precise deposition and epigenetic inheritance during replication. These insights highlight CAF-1's conformational dynamics upon histone binding, which modulate its affinity for PCNA and promote faithful transmission of parental histone modifications to daughter strands.

Chaperone Involvement

Histone chaperones play a crucial role in nucleosome assembly by binding soluble , preventing their nonspecific aggregation, and guiding their sequential deposition onto without becoming integral components of the final nucleosome structure. These proteins ensure the fidelity of formation during processes such as and repair, maintaining epigenetic information and integrity. Chaperones are categorized based on their histone specificity: those dedicated to H3-H4, such as ASF1 and CAF-1, and those for H2A-H2B, including NAP-1 and FACT. ASF1 binds the H3-H4 heterodimer through with the alpha N helix of , stabilizing the dimer and facilitating its delivery to form the tetramer core. This binding prevents histone aggregation and promotes ordered deposition, while CAF-1 further integrates the H3-H4 tetramer into nascent . For H2A-H2B, NAP-1 sequesters dimers to eliminate competing non-nucleosomal histone-DNA interactions, enabling efficient dimer addition to the H3-H4 core. FACT, a heterodimer of Spt16 and Pob3 (or SSRP1 in humans), similarly chaperones H2A-H2B and supports both assembly and disassembly by modulating interactions during dynamic transactions. Specificity distinguishes chaperone functions: CAF-1 is specialized for replication-coupled assembly, associating with the PCNA sliding clamp to deposit histones onto newly synthesized DNA strands. In contrast, NAP-1 operates more generally, facilitating nucleosome assembly in diverse contexts beyond replication, such as transcription-associated events. ASF1 and FACT exhibit broader roles but cooperate with these partners; for instance, ASF1 hands off H3-H4 to CAF-1 during replication. Recent structural advances, including cryo-EM studies from 2024-2025, have revealed transient chaperone-nucleosome intermediates, such as FACT-bound hexasome-like states during assembly, highlighting dynamic, stepwise interactions that preserve nucleosome integrity. These chaperones are evolutionarily conserved across eukaryotes, from yeast to humans, underscoring their essential role in stability by preventing chromatin defects that could lead to replication stress or mutations. Depletion of ASF1, CAF-1, NAP-1, or FACT components results in hypersensitivity to DNA damage and chromosomal instability, emphasizing their non-redundant contributions to faithful nucleosome biogenesis.

Dynamics

Nucleosome Sliding and Positioning

Nucleosome sliding refers to the repositioning of the histone octamer along the DNA, enabling dynamic adjustments in chromatin structure that influence DNA accessibility. This process encompasses both passive and active modes, where the nucleosome translocates without dissociation from the DNA. Positioning, in turn, describes the stable placement of nucleosomes at specific genomic loci, guided by intrinsic DNA properties and extrinsic factors. These movements are crucial for maintaining chromatin organization, though detailed modulation by remodeling complexes is addressed elsewhere. The sliding mechanism involves coupled rotational and translational shifts of the nucleosome along the DNA axis. Translational movement occurs in discrete steps of approximately 5-10 s, during which the DNA twists around the core in a screw-like fashion, requiring a of about 36° per base pair shift to maintain histone-DNA contacts. Passive sliding is driven by , allowing spontaneous, diffusion-like repositioning without external energy input, often observed under physiological conditions like elevated or . In contrast, active sliding is powered by , primarily facilitated by complexes that propagate DNA twists or bulge propagation to shift the nucleosome directionally. Nucleosome positioning is influenced by DNA sequence motifs that act as signals for exclusion or stabilization. Poly(dA:dT) tracts, rich in adenine-thymine base pairs, strongly disfavor nucleosome formation due to their rigid, straight helical structure, thereby creating nucleosome-free regions particularly at gene promoters. In barrier models, these tracts serve as boundaries that statistically position adjacent nucleosomes by limiting their mobility and promoting ordered arrays. Experimental evidence for sliding dynamics has been provided by fluorescence resonance energy transfer (FRET) and single-molecule techniques, which track nucleosome movements in real time. These studies reveal sliding rates of approximately 1-10 base pairs per second for active processes, with passive rates being significantly slower due to reliance on thermal energy. Recent advances in cryo-electron microscopy (cryo-EM) from 2023 to 2025 have captured sliding intermediates, elucidating transient histone-DNA distortions that facilitate translocation. For instance, structures of remodeler-nucleosome complexes show twisted DNA configurations and partial unwrapping during the sliding cycle, highlighting the molecular basis of these distortions.

DNA Site Exposure and Breathing

The breathing model describes the spontaneous, reversible partial unwrapping and rewrapping of DNA segments from the ends of the nucleosome core particle, typically involving 10-20 base pairs (bp) at each entry and exit site. This favors the wrapped state, with an for unwrapping of approximately 0.2-0.6 at the nucleosome ends under physiological conditions. Such fluctuations arise from and the elastic properties of DNA-histone interactions, without requiring external factors like ATP-dependent remodelers. These unwrapping events result in transient site exposure, rendering approximately 20-30 bp of DNA accessible at the entry and exit regions, which can facilitate binding of transcription factors to otherwise occluded sequences. For instance, transcription factors exploit this exposure to initiate access to promoter regions, as the partially unwrapped DNA segments provide a kinetic window for protein-DNA interactions before rewrapping occurs. The kinetics of breathing occur on millisecond timescales, with unwrapped state lifetimes ranging from 10-50 , enabling rapid equilibrium between wrapped and unwrapped conformations. significantly influences these dynamics; higher salt concentrations (50-100 mM NaCl) promote unwrapping by screening electrostatic interactions between DNA and histones, increasing the probability of the open state by up to twofold compared to low-salt conditions. Single-molecule (smFRET) experiments have provided direct evidence for these partial unwrapping probabilities, revealing that 10-20% of nucleosomes exhibit intermediate unwrapped states at the ends, with probabilities decreasing sharply toward the dyad axis (e.g., ~10% at 27 inward). These measurements, combined with gel-based assays, confirm the site-specific nature of breathing and its role in modulating DNA accessibility. Recent advances in 2025 have integrated cryo-electron microscopy (cryo-EM) data with computational modeling to elucidate the structural basis of unwrapping dynamics, capturing heterogeneous conformations and histone rearrangements during partial DNA dissociation.00060-7) This integrative approach highlights how local histone flexibility contributes to the energy landscape of breathing, offering atomic-level insights into the process.

Nucleosome-Free Regions

Nucleosome-free regions (NFRs), also termed nucleosome-depleted regions (NDRs), represent stable segments of eukaryotic DNA that lack nucleosomes, particularly at gene promoters where they span approximately 140–200 base pairs.00257-8)00769-8) These regions are typically flanked by precisely positioned nucleosomes, including the -1 nucleosome upstream and the +1 nucleosome downstream, which together define a characteristic chromatin architecture that facilitates regulatory access to DNA.00257-8) The formation of NFRs arises through intrinsic and extrinsic mechanisms. Intrinsically, certain DNA sequences act as barriers to nucleosome assembly; for instance, poly(dA:dT) tracts, which are rigid and AT-rich, inherently resist nucleosome wrapping due to their biophysical properties, thereby promoting nucleosome exclusion in promoter regions. Extrinsically, factors such as bound proteins can evict or prevent nucleosome occupancy, maintaining the open configuration of these regions. Temporary nucleosome breathing at the edges of NFRs may contribute to their boundaries but does not account for their persistence. NFRs are detected genome-wide using micrococcal nuclease sequencing (MNase-seq), which reveals regions of hypersensitivity to nuclease digestion due to low nucleosome occupancy and high chromatin accessibility. In these assays, NFRs appear as valleys of protection in nucleosome occupancy profiles, contrasting with the periodic peaks from wrapped DNA elsewhere. Biologically, NFRs are crucial for transcription initiation, providing an accessible platform for transcription factors and the pre-initiation complex to bind upstream of the transcription start site, with the downstream +1 nucleosome helping to define the precise positioning for polymerase entry. Recent studies have linked NFRs to broader chromatin organization, showing that their presence contributes to conformationally defined heterogeneous packing domains through interactions with transcription and nucleosome dynamics.

Modulation and Remodeling

Histone Post-Translational Modifications

Histone post-translational modifications (PTMs) occur primarily on the N-terminal tails of core histones H2A, H2B, H3, and H4, as well as on their globular domains, altering the nucleosome's interaction with DNA and other proteins. These covalent changes include acetylation, methylation, phosphorylation, and ubiquitination, each mediated by specific enzymes that add or remove the modifications to regulate chromatin structure. Acetylation involves the addition of acetyl groups to lysine residues by histone acetyltransferases (HATs), such as p300/CBP, neutralizing the positive charge and reducing the electrostatic affinity of histone tails for negatively charged DNA, which loosens nucleosome packing and promotes a more open chromatin conformation. Deacetylation is catalyzed by histone deacetylases (HDACs), restoring the charge and tightening interactions. Key sites include H3K9 and H4K16, where acetylation at H4K16 disrupts internucleosomal contacts, facilitating chromatin fiber decompaction. Methylation adds methyl groups to lysine or arginine residues via histone methyltransferases (HMTs), such as SET1 for H3K4 or EZH2 for H3K27, and can be mono-, di-, or tri-methylated, influencing nucleosome stability without altering charge. For instance, H3K4me3 correlates with euchromatin openness by recruiting reader proteins that stabilize accessible structures, while H3K27me3 compacts chromatin through enhanced tail interactions with adjacent nucleosomes. Demethylation is performed by enzymes like LSD1 or JMJD family members. Phosphorylation adds phosphate groups to serine, threonine, or tyrosine residues by kinases, introducing negative charge that can repel DNA or recruit effectors, altering nucleosome positioning. Ubiquitination conjugates ubiquitin to lysines, often monoubiquitination on H2B K120, which sterically hinders tail folding and modulates nucleosome array compaction.00115-6) The "histone code" hypothesis posits that combinations of these PTMs form a combinatorial interpreted by chromatin-associated factors, dictating nucleosome and accessibility beyond individual marks. Crosstalk between modifications ensures specificity; for example, H3K4 sterically or allosterically blocks H3K9 by competing for the same or reader domains, preventing repressive compaction at active loci.00115-6) Recent studies (2023–2025) highlight how PTMs on non-canonical nucleosomes, such as those with histone variants, expand structural diversity by modulating enzyme access and tail dynamics in specialized chromatin contexts. Histone variants can subtly alter modification sites, influencing PTM patterns as detailed in subsequent sections.

Histone Variants and Non-Canonical Forms

Histone variants are sequence-divergent isoforms of the core that replace their counterparts within nucleosomes, thereby conferring specialized structural and functional properties to . histone H3.1 is primarily incorporated in a replication-coupled manner during to maintain integrity post-DNA synthesis, whereas the variant H3.3 is deposited in a replication-independent fashion, predominantly at transcriptionally active regions to facilitate . Similarly, H2A.Z marks nucleosomes in dynamic environments, such as promoter and enhancer regions prone to frequent remodeling, while CENP-A specifically localizes to centromeric nucleosomes to establish epigenetic identity. These variants differ from histones by only a few but elicit profound effects on nucleosome stability and accessibility. Structurally, H2A.Z introduces alterations in the that reduce DNA wrapping affinity, leading to more unstable nucleosomes with increased propensity for partial unwrapping and histone exchange compared to canonical H2A-containing nucleosomes. For instance, the extended acidic in H2A.Z enhances interactions with remodeling factors while destabilizing the DNA-histone interface, promoting open chromatin conformations. In contrast, CENP-A replaces in centromeric nucleosomes, featuring a unique targeting domain that recruits proteins like CENP-C and CENP-N, thereby directing the assembly of the mitotic machinery essential for chromosome segregation. Post-translational modifications on these variants can further enhance their specificity, such as on H3.3 to promote binding. Non-canonical nucleosomes arise from incomplete histone octamers or variant incorporations, including hexasomes (lacking one H2A-H2B dimer), tetrasomes (lacking both H2A-H2B dimers), and variant-specific octamers, all of which exhibit altered stability to facilitate transient DNA exposure. Hexasomes, for example, display asymmetric DNA wrapping and reduced thermal stability, enabling ATP-dependent remodelers like INO80 to reposition them during transcription initiation. These forms represent intermediate states in nucleosome disassembly or assembly, with variant-octamers such as those containing H2A.Z showing heightened lability that correlates with faster histone turnover rates. The incorporation of histone variants relies on dedicated chaperones that ensure precise deposition. For H3.3, the chaperone DAXX, often in complex with , specifically recognizes and delivers H3.3-H4 dimers to target loci like telomeres and pericentromeric , bypassing replication timing. This chaperone-mediated pathway contrasts with replication-coupled assembly of canonical , allowing H3.3 to maintain active or repressed states independently of . Recent structural studies have illuminated the diversity of non-canonical nucleosomes in overcoming transcription barriers, revealing how hexasomes and variant-octamers enable progression by transiently exposing DNA while preserving overall integrity. Cryo-EM analyses in 2025 have further detailed subnucleosome intermediates involving CENP-A, underscoring their role in maturation beyond canonical centromeric functions.

ATP-Dependent Remodeling Complexes

ATP-dependent chromatin remodeling complexes are multi-subunit enzymes that utilize the energy from ATP hydrolysis to reposition, evict, or exchange nucleosomes, thereby regulating DNA accessibility for processes such as transcription and replication. These complexes are classified into four major families based on the structure of their central ATPase subunit: SWI/SNF, ISWI, CHD, and INO80. The SWI/SNF family, including yeast Swi/Snf and mammalian BAF and PBAF complexes, primarily facilitates nucleosome sliding and eviction to promote gene activation. In contrast, ISWI complexes, such as ACF and CHRAC, specialize in nucleosome spacing and assembly, creating regularly spaced arrays that maintain chromatin organization. CHD family remodelers, like Chd1 and NuRD, focus on nucleosome repositioning and are often involved in transcriptional repression or elongation. The INO80 family, encompassing INO80 and SWR1 complexes, excels in histone variant exchange and nucleosome eviction, particularly at sites of DNA damage or promoters. The core mechanism of these complexes involves the ATPase domain, which belongs to the SNF2 superfamily, binding to nucleosomal DNA and using ATP hydrolysis to translocate the DNA relative to the histone octamer, thereby disrupting histone-DNA contacts. This translocation is thought to proceed via the "bulge propagation" model, where the ATPase induces a bulge or loop in the DNA at the entry site to the nucleosome; propagation of this bulge along the nucleosome surface shifts the DNA by several base pairs, effectively sliding or restructuring the nucleosome. Each cycle of ATP binding, hydrolysis, and release typically advances the DNA by approximately 1-2 base pairs, with the process being directional (often 3' to 5' along one DNA strand) and coupled to conformational changes in the ATPase's RecA-like lobes. These complexes act on both canonical nucleosomes containing histone H3.1/H3.2 and variant nucleosomes, such as those with H2A.Z or H3.3, allowing targeted remodeling at specific genomic loci. A prominent example is the SWR1 complex, which catalyzes the ATP-dependent exchange of canonical H2A-H2B dimers for H2A.Z-H2B dimers in nucleosomes, preferentially at promoter regions to facilitate transcription ; this process involves sequential steps for dimer eviction and insertion without net nucleosome disassembly. Recent studies have revealed that ATP-dependent remodelers cooperate with loop extrusion factors, such as , to define heterogeneous nanoscopic packing domains (~100-500 nm), where remodeling activity modulates local nucleosome density and influences patterns in three-dimensional nuclear space.

Genome-Wide Dynamics and DNA Defects

Genome-wide mapping techniques such as and have revealed periodic nucleosome positioning patterns across eukaryotic genomes, highlighting the organized arrangement of nucleosomes in . involves partial digestion of with micrococcal , which preferentially cleaves , allowing high-resolution profiling of nucleosome occupancy and positioning from sequencing reads of protected DNA fragments. Similarly, assesses accessibility by transposase-mediated insertion of tags into open regions, indirectly delineating nucleosome positions through protection of wrapped DNA and exposure of linkers or depleted areas. These methods demonstrate that nucleosomes often exhibit phased arrays with repeat lengths of approximately 160-200 base pairs, influenced by underlying DNA sequence preferences and regulatory elements. In yeast models, such as , genome-wide analyses uncover dynamic nucleosome remodeling characterized by high turnover rates, particularly at promoters where nucleosomes are rapidly repositioned to facilitate gene activation. Studies using time-resolved nucleosome occupancy show that promoter regions display fluctuating nucleosome configurations, with remodelers driving and reassembly in response to environmental cues, leading to transient exposure of binding sites. For instance, at the PHO5 promoter, nucleosome dynamics involve sequential remodeling events that alter array stability, contributing to a broader of high-mobility nucleosomes genome-wide. This turnover is more pronounced in yeast compared to higher eukaryotes, reflecting a compact organization adapted for rapid transcriptional responses. Nucleosome spacing varies across species, with yeast exhibiting tighter packing than mammals, which influences overall chromatin compaction and accessibility. In S. cerevisiae, the average nucleosome repeat length is about 165 base pairs, comprising 147 base pairs of wrapped DNA and a 18-base-pair linker, enabling dense arrays suited to the smaller genome. In contrast, mammalian cells show longer linkers averaging 40-60 base pairs, resulting in repeat lengths of 180-200 base pairs and more variable positioning that accommodates diverse regulatory demands. These differences arise from evolutionary adaptations in linker histone incorporation and remodeler activity, with yeast relying less on H1 variants for spacing control. DNA twist defects, arising from mismatches in helical phasing between nucleosomal DNA and linker regions, introduce strain that affects nucleosome positioning and mobility. These defects occur when the ~10.5 base pairs per turn of free DNA adjusts to the histone octamer's geometry, which over-twists DNA at certain superhelix locations, propagating as localized kinks or bulges. Such strain facilitates nucleosome sliding by allowing twist propagation around the histone core, influencing genome-wide positioning signals and enabling responses to remodeling forces without full disassembly. In non-canonical nucleosome forms, twist defects are exacerbated, leading to altered conformations that impact chromatin higher-order structures. Recent studies in 2025 have elucidated the interplay between nucleosome remodeling, transcription, and loop in shaping conformation at the genome scale. Integrating computational modeling with experimental data, these works show that coordinated remodeling and extrusion activities generate heterogeneous nanoscopic packing domains, where nucleosome arrays dynamically adjust to transcriptional . This emergent system highlights how twist defects in non-canonical nucleosomes contribute to conformational flexibility, particularly in regulatory hotspots, advancing understanding of large-scale dynamics.

Biological Functions

Role in Gene Transcription

Nucleosomes act as barriers to (RNAPII) transcription by wrapping DNA, which impedes the enzyme's access to promoter regions and progression along the gene body. This barrier function is particularly pronounced at transcription initiation, where nucleosomes must be repositioned or evicted to allow RNAPII assembly into pre-initiation complexes. Studies using single-molecule techniques have shown that RNAPII pauses at nucleosomal boundaries, requiring energy-dependent mechanisms to proceed, as demonstrated in assays with reconstituted templates. In promoter architecture, nucleosome-free regions (NFRs) upstream of the transcription start site (TSS) facilitate RNAPII , while the positioned +1 nucleosome downstream of the TSS precisely defines the start site by occluding alternative initiation points. Genome-wide mapping in reveals that NFRs are enriched with AT-rich sequences that resist nucleosome formation, positioning the +1 nucleosome to modulate promoter clearance and ensure accurate TSS selection. This organization is conserved across eukaryotes, with the +1 nucleosome often exhibiting rotational phasing that influences RNAPII pausing and productive elongation. During transcription , the FACT complex plays a central role by temporarily disassembling nucleosomes ahead of RNAPII and reassembling them behind, preventing stable barriers to progression. FACT binds to H2A-H2B dimers, facilitating their eviction during elongation and redeposition post-passage, as evidenced by biochemical assays showing enhanced RNAPII processivity on nucleosomal templates in the presence of FACT. This dynamic nucleosome management is essential for efficient , with FACT associating with elongating RNAPII . Epigenetic modifications on nucleosomal histones further regulate transcription by altering accessibility. of at 4 (H3K4ac) is enriched at active promoters, promoting an open state that facilitates RNAPII and by recruiting bromodomain-containing factors. In contrast, trimethylation of H3 at 27 (H3K27me3), mediated by Polycomb repressive complex 2, compacts nucleosomes and blocks RNAPII access, enforcing transcriptional repression at developmental genes. These marks often coexist in bivalent domains, balancing poised states for rapid activation or silencing. Recent studies highlight the role of non-canonical nucleosomes, such as tetrasomes and hexasomes, in overcoming transcription barriers. These subnucleosomal particles, formed during RNAPII passage, exhibit structural flexibility that allows polymerase traversal without full disassembly, as revealed by cryo-EM structures of RNAPII on H3-H4 tetrasomes. In , heterogeneous non-canonical forms predominate , enabling dynamic responses to transcriptional demands and integrating with remodeling factors for barrier resolution.

Involvement in DNA Replication and Repair

During DNA replication, nucleosomes positioned ahead of the advancing replication fork must be disassembled to permit the replisome to access and unwind the underlying DNA. ATP-dependent chromatin remodeling complexes, including the ACF (ATP-utilizing chromatin assembly and remodeling factor) and RSF (remodeling and spacing factor), play key roles in this disassembly process by mobilizing or evicting histones, thereby facilitating fork progression. Behind the fork, newly synthesized DNA is rapidly repackaged into nucleosomes through replication-coupled assembly mediated primarily by the chromatin assembly factor 1 (CAF-1) complex, which deposits histone H3-H4 dimers in a PCNA-dependent manner to restore chromatin structure. This coordinated disassembly and reassembly ensures efficient duplication of the genome while maintaining chromatin integrity. A critical aspect of replication is the preservation of epigenetic information through the recycling of parental histones. Modified parental H3-H4 tetramers are randomly but symmetrically segregated to the two daughter DNA strands, allowing post-translational marks to be transferred and inherited, which supports stable epigenetic memory across cell divisions. This process involves histone chaperones that facilitate the redeposition of parental histones alongside newly synthesized ones, preventing dilution of epigenetic states during chromatin duplication. In DNA repair, nucleosomes at damage sites undergo dynamic remodeling, including histone exchange, to expose lesions for repair machinery access. For instance, at double-strand breaks (DSBs), the INO80 chromatin remodeling complex promotes histone exchange by evicting H2A.Z-containing nucleosomes and replacing them with canonical H2A, which facilitates subsequent repair steps. Additionally, the histone variant H2AX within nucleosomes is rapidly phosphorylated at serine 139 (forming γH2AX) by kinases such as ATM and DNA-PK, generating signaling platforms that recruit repair factors like 53BP1 and BRCA1 to amplify the DNA damage response. Nucleosome eviction is particularly important at DSBs to enable repair pathway choice and execution. Eviction of nucleosomes near the break exposes DNA ends, promoting access for (NHEJ) in or (HR) in S/, with remodeling complexes like INO80 and aiding in this transient opening. Recent structural studies have elucidated the mechanism of CAF-1 in replication-coupled assembly, revealing how its subunits bind acetylated H3-H4 and interact with PCNA to deposit nucleosomes efficiently behind the fork, with implications for both replication fidelity and repair contexts.

Contribution to Chromosome Condensation and Stability

Nucleosomes play a central role in condensation by facilitating the higher-order folding of into loops and scaffolds, particularly during mitosis, which compacts the to enable efficient . This process involves the , which binds to nucleosome and promotes the compaction of nucleosome arrays into 30-nm fibers and further structures, stabilizing interactions that reduce volume by orders of magnitude. Similarly, (HP1) contributes by binding to tails methylated at lysine 9 (H3K9me), bridging adjacent nucleosomes to form condensed domains essential for mitotic architecture. These mechanisms ensure that chromosomes achieve the necessary density for attachment and movement without entanglement. Beyond condensation, nucleosomes enhance chromosome stability by preventing DNA tangling and shielding the genome from nuclease degradation. The wrapping of DNA around histone octamers constrains supercoiling and reduces the risk of knots or catenanes during chromosome segregation, maintaining structural integrity across cell divisions. Additionally, nucleosome assembly inherently protects DNA from endonucleases, as the core particle sequesters approximately 146 base pairs of DNA, rendering it inaccessible to degradative enzymes and thereby safeguarding genomic material . At centromeres, specialized nucleosomes containing the histone H3 variant CENP-A serve as the epigenetic foundation for assembly, enabling precise attachment during . CENP-A nucleosomes recruit constitutive centromere-associated network (CCAN) proteins, which in turn stabilize the inner and facilitate bioriented alignment, a process critical for error-free segregation. This variant replaces canonical in centromeric , altering nucleosome stability to support the dynamic yet robust interactions required for function. In , nucleosomes incorporating the histone variant H3.3 contribute to end protection by maintaining heterochromatic structures that prevent DNA damage and fusions. The deposition of H3.3 at telomeric repeats, mediated by chaperones like ATRX-DAXX, promotes trimethylation of H3 lysine 9 (), which recruits protective factors and suppresses recombination, thereby preserving telomere length and chromosome stability over multiple divisions. Recent studies have highlighted how nucleosome spacing influences higher-order assembly and stability through mechanisms. Variations in length, tunable at single base-pair resolution, modulate internucleosomal interactions to either promote or inhibit liquid-like , fine-tuning compaction and resistance to mechanical stress in mitotic chromosomes. Furthermore, phase-separated domains involving nucleosomes and associated proteins enhance overall stability by compartmentalizing , reducing diffusion, and protecting against genotoxic insults.

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