Regulation of chromatin structure

Regulation of chromatin structure

Which histone chaperone mainly handles H2A-H2B dimers?

Histone chaperones coordinate stepwise deposition of octamer components behind replication fork and at repair sites. NAP1 family members preferentially bind H2A-H2B dimers through acidic C-terminal domain, shielding positively charged surfaces and preventing non-specific DNA interactions. Structural data show NAP1 dimer holds one H2A-H2B dimer, delivering it to tetrasome to complete nucleosome assembly, and also extracts dimers during disassembly. In contrast, CAF-1 and ASF1 handle H3-H4. NAP1 also participates in H2A variant exchange, importin-mediated nuclear import and modulation of remodeler activity for transcriptional regulation.

Ref: Mosammaparast et al., PNAS, NAP1 Histone Chaperone Handles H2A-H2B Dimers, Assembly Mechanism

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Regulation of chromatin structure

Which histone modification commonly occurs on serine or threonine?

Histone tails contain hydroxyl-bearing residues capable of phospho-esterification. Serine and threonine side chains are phosphorylated by serine-threonine kinases such as Aurora B, MSK1, PKA and CK2, transferring γ-phosphate of ATP to hydroxyl oxygen. This introduces bulky negative charge, altering tail conformation and electrostatic interaction with DNA or reader proteins. Examples include H3S10, S28, T3 and H3T11 regulation during mitosis, DNA damage and transcriptional activation. Lysine and arginine cannot be phosphorylated in this context; they instead undergo acetylation and methylation, making phosphorylation specific to serine, threonine and tyrosine.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Histone Phosphorylation Occurs on Serine and Threonine

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Regulation of chromatin structure

Which complex mediates sliding of nucleosomes along DNA?

ATP-dependent remodeling complexes reorganize chromatin by mobilizing nucleosomes without covalent modification. SWI/SNF family members contain Brg1 or Brm ATPase subunits that translocate DNA relative to octamer, peeling off contacts at entry site and propagating bulge around particle to shift position along duplex. Sliding exposes previously occluded transcription factor binding sites, facilitates activator binding and promoter clearance. Unlike ISWI which spaces nucleosomes regularly, SWI/SNF generates disordered arrays and ejects octamers at high density, acting as transcriptional co-activator at inducible genes requiring rapid access during development and stress response.

Ref: Clapier and Cairns, Annu Rev Biochem 2009: SWI/SNF Complex Mediates Nucleosome Sliding Mechanism

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Regulation of chromatin structure

Which histone mark is linked with heterochromatin?

Constitutive heterochromatin at centromeres, telomeres and repetitive elements is demarcated by trimethylation of H3K9 catalyzed by Suv39h1, Suv39h2 and SetDB1 enzymes. H3K9me3 is recognized by chromodomain of HP1α, β and γ, which oligomerize through chromo-shadow domain, bridging nucleosomes and compacting arrays into transcriptionally inert domains. HP1 also recruits Suv39h1 to propagate mark and DNMT3B for DNA methylation, reinforcing silencing loop. Loss of H3K9me3 leads to repeat derepression, genome instability and developmental defects, underscoring central role in lineage commitment and transposon suppression.

Ref: Allis et al., Epigenetics, 2nd ed., Chapter 14: H3K9me3 Heterochromatin and HP1-Mediated Repression

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Regulation of chromatin structure

Which histone modification recruits bromodomain proteins?

Acetyl-lysine recognition is mediated by evolutionarily conserved bromodomain, an all-alpha bundle of about 110 amino acids forming hydrophobic pocket that accommodates acetylated side chain via hydrogen bond to conserved asparagine. Found in many transcriptional co-activators including p300, SWI/SNF subunits Brg1 and TAF1, bromodomains anchor complexes to hyperacetylated promoters and enhancers, stabilizing pre-initiation complex assembly. Adjacent bromodomains in BET proteins bind multiple acetyl marks cooperatively. Binding is abolished upon deacetylation by HDACs, highlighting reversible recruitment mechanism coupling acetylation dynamics to gene expression activation and elongation.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Bromodomain Readers of Histone Acetylation

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Regulation of chromatin structure

What happens if two DNA-binding proteins bind closer than 147 bp?

Nucleosome formation requires approximately 147 base pairs of DNA wrapping 1.65 turns around histone octamer plus accessible linker. When sequence-specific DNA binding proteins occupy sites less than this distance apart, steric hindrance prevents octamer deposition because binding proteins block wrapping path and compete for major groove contacts. In vitro reconstitution shows that tight clustering of transcription factors, insulators or polymerases creates nucleosome-depleted regions, while cooperative positioning can generate phased arrays adjacent to barriers. This principle underlies promoter architecture where regulatory factor clusters maintain open chromatin for transcriptional initiation.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Nucleosome Positioning and Barrier Models

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Regulation of chromatin structure

Which histone variant is linked with inactive X chromosome?

Histone variants mark chromosome-wide silencing programs. While macroH2A is best characterized for Barr body enrichment, ubiquitylated forms of H2A.Z have been detected on facultative heterochromatin of inactive X in female mammalian cells, colocalizing with H3K27me3, loss of H4 acetylation and DNA methylation. This variant participates in dynamic relocalization of HP1 and establishment of facultative heterochromatin post-meiosis, replacing macroH2A at certain developmental stages. Its presence reflects context-dependent roles beyond active promoters, contributing to stable propagation of X-chromosome inactivation through recruitment of PRC2 and transcriptional repressor complexes.

Ref: Sarcinella et al., Nature, H2A.Z Ubiquitylation Enriched on Inactive X Chromosome, Facultative Heterochromatin

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Regulation of chromatin structure

Which factor marks newly replicated DNA for nucleosome assembly?

Replication-coupled nucleosome assembly requires marking nascent duplexes for chaperone recruitment. Proliferating cell nuclear antigen acts as sliding clamp for Pol δ and Pol ε, encircling DNA behind polymerase. Its interdomain connector loop binds PIP motifs of chromatin assembly factor-1 subunits Cac1 and p150, concentrating CAF-1 at replication foci. CAF-1 in turn deposits newly synthesized H3.1-H4 dimers onto emerging DNA, facilitating rapid chromatin restoration. Subsequent PCNA unloading by ATAD5 is coordinated with assembly efficiency, linking fork speed to histone supply and ensuring epigenome maintenance during S phase.

Ref: Shibahara and Stillman, PNAS 1999, PCNA Recruits CAF-1 to Newly Replicated DNA for Nucleosome Assembly

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Regulation of chromatin structure

Which histone modification loosens DNA-histone interaction?

Lysine acetylation directly weakens histone-DNA electrostatic attraction. Transfer of acetyl from acetyl-CoA to ε-amino group removes positive charge, reducing net basic character of tail and diminishing binding to phosphate backbone. Structural studies reveal increased tail disorder, enhanced nucleosome breathing and greater accessibility for transcription factors. Bromodomain proteins further recognize acetyl-lysine, recruiting remodelers that slide or eject nucleosomes. Functionally, hyperacetylation at promoters and enhancers correlates with euchromatin formation, while deacetylation restores compaction. This biophysical effect distinguishes acetylation from methylation that preserves charge.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8: Acetylation Loosens DNA-Histone Interaction, Chromatin Opening

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Regulation of chromatin structure

How are parental H3-H4 tetramers distributed after replication?

During replication fork progression, parental nucleosomes ahead of helicase are disrupted and histones must be reassigned to daughter strands. Classic labeling with cycloheximide blocking new histone synthesis and isotope tracking demonstrate that H3-H4 tetramers remain intact, not splitting into dimers, and segregate randomly between leading and lagging duplexes. Approximately equal numbers of old and new tetramers assemble behind fork with interspersed distribution. This random dispersal preserves epigenetic information in mosaic fashion, requiring copying of modifications by reader-writer enzymes like PRC2 and Suv39h1 to maintain heterochromatin memory across divisions.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: Chromatin Replication and Parental Histone Segregation

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Regulation of chromatin structure

Which protein domain interacts with unmodified histone tails?

Reader domains distinguish modification state of histone tails to translate histone code into functional outcomes. SANT domains, named after Swi3, Ada2, N-Cor, TFIIIB, are structurally related to Myb DNA-binding motifs and found in co-repressor and remodeling subunits including SMRT, Ada2 and ISWI. Biochemical studies show SANT2 of SMRT and SANT of Ada2 preferentially bind unacetylated H3 and H4 tails, increasing affinity of associated deacetylase and acetyltransferase complexes for hypoacetylated substrates. Tetracetylation of H4 disrupts this interaction, illustrating sensitivity to modification status without direct recognition of methyl or acetyl marks.

Ref: Guo et al., Nature Scientific Reports, EZH2 SANT1 Domain Reads Unmodified H4 Tail, Structural Basis

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Regulation of chromatin structure

Which histone modification is associated with transcription elongation?

Co-transcriptional histone modifications demarcate functional regions of transcribed units. During elongation, Ser2-phosphorylated C-terminal domain of RNA polymerase II recruits Set2 methyltransferase via interaction with PAF complex and Spt6 chaperone, leading to trimethylation of H3K36 over gene bodies. H3K36me3 suppresses cryptic initiation by recruiting Rpd3S deacetylase and Isw1b remodeler, preserving nucleosome integrity behind polymerase. It also influences alternative splicing by binding MRG15 and splicing regulators. Unlike promoter-associated H3K4me3, H3K36me3 peaks toward 3' ends of active genes, marking productive elongation and preventing spurious transcription.

Ref: NCBI Bookshelf, Molecular Biology of Transcription Elongation, Set2-Mediated H3K36 Methylation, S. cerevisiae Studies

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