Chemical activation of Sir2-dependent silencing by relief of nicotinamide inhibition

Sir2 is a nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacetylase involved in gene silencing and longevity. Cellular stresses affect Sir2 activity, but the mechanisms of Sir2 regulation are debated. Nicotinamide has been proposed as a physiological regulator that inhibits Sir2 deacetylase activity by chemical reversal of a covalent reaction intermediate. We demonstrate a chemical strategy to activate Sir2-dependent transcriptional silencing and present evidence that the endogenous level of nicotinamide limits Sir2 activity in wild-type (wt) yeast cells. Nicotinamide inhibition of Sir2 is antagonized in vitro by isonicotinamide, which causes an increase in Sir2 deacetylation activity. Isonicotinamide also substantially increases transcriptional silencing at Sir2-regulated loci in wt strains and in strains lacking key NAD⁺ salvage pathway enzymes (PNC1 and NPT1). Thus, a nicotinamide antagonist is a Sir2 agonist in vitro and in vivo.