Practice question

Histone acetylation neutralizes lysine epsilon-amino positive charge weakening DNA-histone interaction relaxing chromatin structure facilitating transcription factor access. Reversal mediated by histone deacetylases HDACs divided into Rpd3, Hda1, Sir2 families uses zinc or NAD as cofactors restoring positive charge promoting nucleosome compaction and repressive chromatin assembly. During glucose repression Tup1-recruited HDACs Rpd3L, Hda1 deacetylate H3K9, H3K18, H4K16 at GAL promoters preventing SAGA acetyltransferase Gcn5 access. Similarly at Saccharomyces telomeres Sir2 NAD-dependent HDAC deacetylates H4K16 enabling Sir3/Sir4 spreading and telomere position effect. HDAC activity antagonizes HATs establishing dynamic acetylation equilibrium regulating gene expression and heterochromatin maintenance precisely.