Chromatin assembly factor Asf1p-dependent occupancy of the SAS histone acetyltransferase complex at the silent mating-type locus HMLα

Transcriptional repression of the silent mating-type loci HMLα and HMRa in Saccharomyces cerevisiae is regulated by chromatin structure. Sas2p is a catalytic subunit of the SAS histone acetyltransferase (HAT) complex. Although many HATs seem to relieve chromosomal repression to facilitate transcriptional activation, sas mutant phenotypes include loss of SIR1-dependent silencing of HMLα. To gain insight into the mechanism of the SAS complex mediated silencing at HMLα, we investigated the expression and chromatin structure of the α2 gene in the HMLα locus. We found that deletion of SAS2 in combination with a null allele of SIR1 changed the chromatin structure of the precisely positioned nucleosome, which includes the mRNA start site of the α2 gene and derepressed α2 transcription. The Sas2p HAT domain was required for this silencing. Furthermore, chromatin immunoprecipitation analysis revealed that the SAS complex was associated with the HMLα locus, and ASF1 (which encodes chromatin assembly factor Asf1p), but not SIR1 and SIR2, was necessary for this localization. These data suggest that the HAT activity and ASF1-dependent localization of the SAS complex are required for SIR1-dependent HMLα silencing.