Nucleosome Assembly Factors CAF-1 and HIR Modulate Epigenetic Switching Frequencies in an H3K56 Acetylation-Associated Manner in Candida albicans

Abstract

CAF-1 and HIR are highly conserved histone chaperone protein complexes that function in the assembly of nucleosomes onto chromatin. CAF-1 is characterized as having replication-coupled nucleosome activity, whereas the HIR complex can assemble nucleosomes independent of replication. Histone H3K56 acetylation, controlled by the acetyltransferase Rtt109 and deacetylase Hst3, also plays a significant role in nucleosome assembly. In this study, we generated a set of deletion mutants to genetically characterize pathway-specific and overlapping functions of CAF-1 and HIR in C. albicans. Their roles in epigenetic maintenance of cell type were examined by using the white-opaque switching system in C. albicans. We show that CAF-1 and HIR play conserved roles in UV radiation recovery, repression of histone gene expression, correct chromosome segregation, and stress responses. Unique to C. albicans, the cac2⌬/⌬ mutant shows increased sensitivity to the Hst3 inhibitor nicotinamide, while the rtt109⌬/⌬ cac2⌬/⌬ and hir1⌬/⌬ cac2⌬/⌬ mutants are resistant to nicotinamide. CAF-1 plays a major role in maintaining cell types, as the cac2⌬/⌬ mutant exhibited increased switching frequencies in both directions and switched at a high frequency to opaque in response to nicotinamide. Like the rtt109⌬/⌬ mutant, the hir1⌬/⌬ cac2⌬/⌬ double mutant is defective in maintaining the opaque cell fate and blocks nicotinamide-induced opaque formation, and the defects are suppressed by ectopic expression of the master white-opaque regulator Wor1. Our data suggest an overlapping function of CAF-1 and HIR in epigenetic regulation of cell fate determination in an H3K56 acetylation-associated manner.

Differential Regulation of White-Opaque Switching by Individual Subunits of Candida albicans Mediator

“…S1 in the supplemental material). An alternative explanation supported by previous studies is that there are specific chromatin structures associated with both epigenetic transcriptional states and that mutations which impair the formation of these structures can impair the stability of one or both states (50,52). Previous studies of the med16⌬ mutation in S. cerevisiae support the idea that Med16 could impact the stability of chromatin structures and epigenetic states (22,26), although the mechanism is still unclear.…”

“…The only other two mutations found to have a similar impact are deletion of CAC2, which is part of the CAF-1 complex that functions mainly in replicationcoupled nucleosome assembly (52), and deletion of RPD3, a histone deacetylase associated with transcriptional repression (49). With the exception of the increased EFG1 expression in med16⌬/⌬ opaque cells, there appear to be no significant changes in the expression levels of the key components of the Wor1 circuit (Fig.…”

“…Though many transcription networks utilize positive-feedback loops (48), few of them have documented heritably stable transcriptional states similar to those of the Wor1-driven circuit. Several recent studies have identified a key role for chromatin structure and dynamics in white-opaque switching (49)(50)(51)(52). Given the connection between Mediator and telomeric heterochromatin and silencing in S. cerevisiae, we sought to determine whether C. albicans Mediator affects the bistability of gene expression in the white and opaque states.…”

Overlapping Functions between SWR1 Deletion and H3K56 Acetylation in Candida albicans

“…In addition, the swr1 mutant showed no increased growth sensitivity to hydroxyurea or heat and osmotic stress (not shown). We have previously shown that the rtt109 mutant has an increased rate of generating aneuploidy (25). However, we did not observe any increase in aneuploidy formation in the swr1 mutant after growth on L-sorbose medium (not shown).…”

“…Treatment with nicotinamide (NAM), which can inhibit sirtuin activity noncompetitively (22,23) and increases H3K56ac levels, also promotes switching to the opaque form, and this activity has also been shown to be dependent on RTT109 (24). Furthermore, hyperacetylation of H3K56 is toxic to C. albicans (21,25).…”