Proliferating Cell Nuclear Antigen (PCNA) Is Required for Cell Cycle-regulated Silent Chromatin on Replicated and Nonreplicated Genes

In Saccharomyces cerevisiae, silent chromatin is formed at HMR upon the passage through S phase, yet neither the initiation of DNA replication at silencers nor the passage of a replication fork through HMR is required for silencing. Paradoxically, mutations in the DNA replication processivity factor, POL30, disrupt silencing despite this lack of requirement for DNA replication in the establishment of silencing. We tested whether pol30 mutants could establish silencing at either replicated or non-replicated HMR loci during S phase and found that pol30 mutants were defective in establishing silencing at HMR regardless of its replication status. Although previous studies tie the silencing defect of pol30 mutants to the chromatin assembly factors Asf1p and CAF-1, we found pol30 mutants did not exhibit a gross defect in packaging HMR into chromatin. Rather, the pol30 mutants exhibited defects in histone modifications linked to ASF1 and CAF-1-dependent pathways, including SAS-I-and Rtt109p-dependent acetylation events at H4-K16 and H3-K9 (plus H3-K56; Miller, A., Yang, B., Foster, T., and Kirchmaier, A. L. (2008) Genetics 179, 793-809). Additional experiments using FLIM-FRET revealed that Pol30p interacted with SAS-I and Rtt109p in the nuclei of living cells. However, these interactions were disrupted in pol30 mutants with defects linked to ASF1- and CAF-1-dependent pathways. Together, these results imply that Pol30p affects epigenetic processes by influencing the composition of chromosomal histone modifications.