THE ROLE OF SAS2, AN ACETYLTRANSFERASE HOMOLOG OF SACCHAROMYCES-CEREVISIAE, IN SILENCING AND ORC FUNCTION

Silencing at the cryptic mating-type loci HML and HMR of Saccharomyces cerevisiae requires regulatory sites called silencers. Mutations in t he Rap1 and Abf1 binding sites of the HMR-E silencer (HMRa-e*) cause the silencer to be nonfunctional, and hence, cause derepression of HMR . Here, we have isolated and characterized mutations in SAS2 as second -site suppressors of the silencing defect of HMRa-e*. Silencing confe rred by tile removal of SAS2 (sas2 Delta) depended upon the integrity of the ARS consensus sequence of the HMR-E silencer, thus arguing for an involvement of the origin recognition complex (ORC). Restoration of silencing by sas2 Delta required ORC2 and ORC5, but not SIR1. or RAP1 . Furthermore, sas2 Delta suppressed the temperature sensitivity, but not the silencing defect of or orc2-1 and orc5-1. Moreover, sas2 Delta had opposing effects on silencing of HML and HMR. The putative Sas2 p rotein bears similarities to known protein acetyltransferases. Several models for the role of Sas2 in silencing are discussed.