Pivotal roles of PCNA loading and unloading in heterochromatin function.

Abstract

In <i>Saccharomyces cerevisiae</i>, heterochromatin structures required for transcriptional silencing of the <i>HML</i> and <i>HMR</i> loci are duplicated in coordination with passing DNA replication forks. Despite major reorganization of chromatin structure, the heterochromatic, transcriptionally silent states of <i>HML</i> and <i>HMR</i> are successfully maintained throughout S-phase. Mutations of specific components of the replisome diminish the capacity to maintain silencing of <i>HML</i> and <i>HMR</i> through replication. Similarly, mutations in histone chaperones involved in replication-coupled nucleosome assembly reduce gene silencing. Bridging these observations, we determined that the proliferating cell nuclear antigen (PCNA) unloading activity of Elg1 was important for coordinating DNA replication forks with the process of replication-coupled nucleosome assembly to maintain silencing of <i>HML</i> and <i>HMR</i> through S-phase. Collectively, these data identified a mechanism by which chromatin reassembly is coordinated with DNA replication to maintain silencing through S-phase.