{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n A growing body of evidence demonstrates that specific post‐translational modifications on the histone H4 tail play central roles in modulating chromatin architecture, DNA repair, transcription regulation, and cell cycle progression. For instance, acetylation of H4 at lysine 16 (H4‐K16Ac) is well documented to impede the formation of compact chromatin fibers and reduce interactions between nucleosomal arrays, thereby facilitating a more open, transcriptionally active chromatin state and affecting the activity of chromatin remodelers."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " In parallel, the methylation status of H4 lysine 20 is critical for DNA repair; recognition of H4 dimethylated at lysine 20 (H4‐K20me2) by tandem Tudor domains of repair factors such as 53BP1 ensures proper targeting of these proteins to double‐strand breaks."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": " The dynamic balance of these modifications is further controlled by specific enzymes: for example, the NAD⁺‐dependent deacetylase SirT2 selectively targets H4‐K16Ac at the G2/M transition to promote chromatin condensation, demonstrating a key mitotic function"}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": ", while TIP60‐dependent acetylation of H4 can modulate the competitive binding of repair versus recombination factors at sites of damage."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": " In addition to these regulatory events, the structural incorporation of histone H4 into specialized nucleosomal complexes is essential for epigenetic identity; for example, CENP‐A/H4 heterotetramers confer a unique rigidity to centromeric nucleosomes that is crucial for centromere specification and stability."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " In Drosophila and mammalian systems, acetylation of H4 by enzymes such as MOF and its subsequent recognition by bromodomain‐containing proteins (e.g. Brdt) underlie key events in dosage compensation and transcriptional activation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " Moreover, shifts in cellular metabolism during stem cell activation can alter the acetylation status of H4—specifically H4‐K16Ac—via modulation of NAD⁺‐dependent deacetylase activities such as that of SIRT1, thereby linking metabolic state to epigenetic control."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "9"}]}, {"type": "t", "text": " Finally, the demethylase activity of PHF8 on H4‐K20me1 further underscores how removal of repressive marks contributes to cell cycle progression and gene activation."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "10"}]}, {"type": "t", "text": " Together, these studies reveal that the diverse modifications found on the H4 tail (often assayed using reagents such as H4C12) serve as dynamic signals that integrate chromatin structure with fundamental cellular processes.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Michael Shogren-Knaak, Haruhiko Ishii, Jian-Min Sun, et al. 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