Pivotal roles of PCNA loading and unloading on heterochromatin function

Abstract

In Saccharomyces cerevisiae, heterochromatin structures required for transcriptional silencing of the HML and HMR loci are duplicated in coordination with passing DNA replication forks. Despite major reorganization of chromatin structure, the heterochromatic, transcriptionally-silent states of HML and HMR are successfully maintained throughout S-phase. Mutations of specific components of the replisome diminish the capacity to maintain silencing of HML and HMR through replication. Similarly, mutations in histone chaperones involved in replication-coupled nucleosome assembly reduce gene silencing. Bridging these observations, we determined that the PCNA unloading activity of Elg1 was important for coordinating DNA replication forks with the process of replication-coupled nucleosome assembly to maintain silencing of HML and HMR through S-phase. Collectively these data identified a mechanism by which chromatin reassembly is coordinated with DNA replication to maintain silencing through S-phase.\n\nSIGNIFICANCE STATEMENTDNA replication poses a unique logistical challenge for the cell in that structural features of chromatin and their regulatory functions must be carefully coordinated with passage of replication machinery so faithful duplication of both the genome and its chromatin structures may be achieved. Nucleosome assembly is fundamental to reestablishment of chromatin in the wake of DNA replication, and here a mechanism by which nucleosome assembly is coordinated with DNA replication to maintain silenced chromatin is described.