The Origin Recognition Complex, SIR1 , and the S Phase Requirement for Silencing

Silencing of transcription in Saccharomyces cerevisiae has several links to DNA replication, including a role for the origin recognition complex (ORC), the DNA replication initiator, in both processes. In addition, the establishment of silencing at the HML and HMR loci requires cells to pass through the S phase of the cell cycle. Passage through S phase was required for silencing of HMR even under conditions in which ORC itself was no longer required. The requirement for ORC in silencing of HMR could be bypassed by tethering the Sir1 protein to the HMR-E silencer. However, ORC had a Sir1-independent role in transcriptional silencing at telomeres. Thus, the role of ORC in silencing was separable from its role in initiation, and the role of S phase in silencing was independent of replication initiation at the silencers.