Practice question

Sir2 founding member of sirtuin family is NAD-dependent class III histone deacetylase central to mating type and telomere silencing in Saccharomyces cerevisiae. Sir2 catalyzes deacetylation of acetyl-lysine producing deacetylated histone, nicotinamide, O-acetyl-ADP-ribose using NAD as co-substrate. Preferred substrates H4K16ac, H3K9ac, H3K14ac removal creates hypoacetylated nucleosomes high-affinity for Sir3 bromo adjacent homology domain and Sir4 binding, enabling spreading of SIR complex. Fission yeast Sir2 similarly deacetylates H3K9ac facilitating Clr4 methylation. Sir2 also deacetylates non-histone proteins linking metabolic NAD levels to epigenetic state and longevity regulation. Its activity opposed by acetyltransferases Sas2, Gcn5 establishing dynamic acetylation equilibrium controlling silencing domains.