The S-cerevisiae SET3 complex includes two histone deacetylases, Hos2 and Hst1, and is a meiotic-specific repressor of the sporulation gene program

Set3 is one of two proteins in the yeast Saccharomyces cerevisiae that, lik e Drosophila Trithorax, contains both SET and PHD domains. We found that Se t3 forms a single complex, Set3C, with Snt1, YIL112w, Sif2, Cpr1, and two p utative histone deacetylases, Hos2 and NAD-dependent Hst1. Set3C includes N AD-dependent and independent deacetylase activities when assayed in vitro. Homology searches suggest that Set3C is the yeast analog of the mammalian H DAC3/SMRT complex. Set3C represses genes in early/middle of the yeast sporu lation program, including the key meiotic regulators ime2 and ndt80. Wherea s Hos2 is only found in Set3C, Hst1 is also present in a complex with Sum1, supporting previous characterizations of Hst1 and Sum1 as repressors of mi ddle sporulation genes during vegetative growth. However, Hst1 is not requi red for meiotic repression by Set3C, thus implying that Set3C (-Hst1) and n ot Hst1-Sum1, is the meiotic-specific repressor of early/middle sporulation genes.