HISTONE DEACETYLASE ACTIVITY OF RPD3 IS IMPORTANT FOR TRANSCRIPTIONALREPRESSION IN-VIVO

Eukaryotic organisms from yeast to human contain a multiprotein comple x that includes Rpd3 histone deacetylase and Sin3 corepressor. The Sin 3-Rpd3 complex, when recruited to promoters by specific DNA-binding pr oteins, can direct transcriptional repression of specific classes of t arget genes. It has been proposed that the histone deacetylase activit y of Rpd3 is important for repression, but direct evidence is lacking. Here, we describe four Rpd3 derivatives with mutations in evolutionar ily invariant histidine residues in a putative deacetylation motif. Th ese Rpd3 mutants lack detectable histone deacetylase activity in vitro , but interact normally with Sin3 in vivo. In yeast cells, these catal ytically inactive mutants are defective for transcriptional repression . They retain some residual Rpd3 function in vivo, however, suggesting that repression by the Sin3-Rpd3 complex may not be attributable excl usively to its intrinsic histone deacetylase activity. Finally, we sho w that a human Rpd3 homolog can interact with yeast Sin3 and repress t ranscription when artificially recruited to a promoter. These results suggest that the histone deacetylase activity of Rpd3 is important, bu t perhaps not absolutely required, for transcriptional repression in v ivo.