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Abstract
The establishment of silencing at the silent mating-type locus, HMR, in Saccharomyces cerevisiae requires that yeast pass through S phase of the cell cycle, yet requires neither the initiation of DNA replication at the locus destined to become silenced nor the passage of a replication fork through that locus. We tested whether this S-phase requirement reflects a window within the cell cycle permissive for recruitment of Sir proteins to HMR. The S-phase-restricted event necessary for silencing occurred after recruitment of Sir proteins to HMR. Moreover, cells arrested in early S phase formed silent chromatin at HMR, provided HMR was on a nonreplicating template. Replicating templates required a later step for silencing. These results provide temporal resolution of discrete steps in the formation of silent chromatin and suggest that more than one cell cycle-regulated event may be necessary for the establishment of silencing.
This work was supported by American Cancer Society postdoctoral fellowship PF-01-126-01-MBC (A.L.K.), USDA Hatch grant IND053072 (A.L.K.), Kimmel Scholar Award SKF-03-010 (A.L.K.), and National Institutes of Health grant GM31105 (J.R.). This project was also supported by an American Cancer Society Institutional Research Grant to the Purdue Cancer Center.
This is paper number 2005-17808 from the Purdue University Agricultural Experiment Station.
We thank Joe Ogas and Scott Briggs for helpful discussions and Marcia Kremer for technical assistance.