Practice question

Chromatin accessibility regulated by chemical state of histone tails altering net charge. Among modifications, lysine acetylation catalyzed by HAT families GCN5, p300, Myst neutralizes epsilon-amino positive charge, weakening histone-DNA electrostatic contacts and disrupting internucleosomal H4 tail-acidic patch interaction essential for 30 nanometer fiber condensation, thereby promoting open euchromatic state. Methylation retains positive charge and can recruit repressors or activators depending context, phosphorylation adds negative charge mainly signaling mitosis or damage rather than general opening, ubiquitination adds bulky 76 amino acid protein altering stability. Therefore acetylation most directly linked to chromatin opening.