THE ORIGIN RECOGNITION COMPLEX, SIR1, AND THE S-PHASE REQUIREMENT FORSILENCING

Silencing of transcription in Saccharomyces cerevisiae has several lin ks to DNA replication, including a role for the origin recognition com plex (ORC), the DNA replication initiator, in both processes. In addit ion, the establishment of silencing at the HML and HMR loci requires c ells 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 silenci ng 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 separa ble from its role in initiation, and the role of S phase in silencing was independent of replication initiation at the silencers.