Promoter Strength Influences the S Phase Requirement for Establishment of Silencing at the Saccharomyces cerevisiae Silent Mating Type Loci

In Saccharomyces cerevisiae, the two cryptic mating type loci, HML and HMR, are transcriptionally silent. Previous studies on the establishment of silencing at HMR identified a requirement for passage through S phase. However, the underlying mechanism for this requirement is still unknown. In contrast to HMR, we found that substantial silencing of HML could be established without passage through S phase. To understand this difference, we analyzed several chimeric HM loci and found that promoter strength determined the S phase requirement. To silence a locus with a strong promoter such as the a1/a2 promoter required passage through S phase while HM loci with weaker promoters such as the alpha 1/alpha 2 or TRP1 promoter did not show this requirement. Thus, transcriptional activity counteracts the establishment of silencing but can be overcome by passage through S phase.