Fission yeast chromatin assembly factor 1 assists in the replication-coupled maintenance of heterochromatin

Chromatin assembly factor-1 (CAF1) is a well-conserved histone chaperone that loads the histone H3-H4 complex onto newly synthesized DNA in vitro through interaction with the replication factor PCNA. CAF1 is considered to be involved in heterochromatin maintenance in several organisms, but the evidence is circumstantial and functional details have not been established. We identified fission yeast CAF-1 (spCAF1), which interacts with PCNA in S phase. Depletion of spCAF1 caused defects in silencing at centromeric and mating locus heterochromatin, accompanied with a decrease in Swi6, the fission yeast HP1 homologue. Loss of spCAF1 destabilized both the silent and active states of chromatin at the meta-stable heterochromatic region, with a more pronounced effect on the silent state, indicating that spCAF1 is involved in the maintenance of heterochromatin. Swi6 dissociated from heterochromatin during G1/S phase appears to associate with spCAF1. In early S phase, spCAF1 localized to replicating heterochromatin as well as euchromatin and remained associated with Swi6, and Swi6 then bound to heterochromatin. Taken together, we propose that spCAF1 functions in heterochromatin maintenance by recruiting dislocated Swi6 during replication to replicated heterochromatin at the replication fork.