#chromatin assembly

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 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

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 histone chaperone deposits H3-H4 during DNA replication?

During S phase newly synthesized histones deposited behind replication fork by dedicated chaperone complexes ensuring rapid nucleosome reassembly and genome stability. CAF-1 complex comprising three subunits p150, p60, p48 specifically binds proliferating cell nuclear antigen PCNA at replication fork and hands H3-H4 dimers to assembling DNA, coupling nucleosome formation to synthesis. NAP1 chaperones H2A-H2B dimers, ASF1 transfers H3-H4 dimers to CAF-1 and HIRA variant pathway, FACT facilitates transcription elongation through nucleosomes allowing polymerase passage. CAF-1 depletion causes S phase delay, heterochromatin defects and replication stress. Thus CAF-1 designated replication dependent H3-H4 chaperone essential for chromatin inheritance.

Ref: Smith and Stillman 1989 CAF-1 Discovery; Alberts et al., Chapter 5: CAF-1 Deposits H3-H4 During Replication

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