Histone variant H2A.Z regulates gene silencing and chromosome segregation in Saccharomyces cerevisiae

Saccharomyces cerevisiae uses a gene repression mechanism known as silencing to regulate the expression of its key developmental genes. Studies have indicated that cell-cycle progression is required for the establishment of silencing; however, the specific cell-cycle events or factors required for establishing silencing are not well understood. Using an inducible silencing system we defined the discrete cell-cycle interval when silencing is established. We find that the establishment of gene silencing occurs during the anaphase-telophase transition of the cell-cycle. Previous studies have shown that H2A.Z acts as a barrier to prevent the spreading of heterochromatin into euchromatin. Studies from our lab have demonstrated that deletion of HTZ1, the gene coding for H2A.Z, eliminates the cell-cycle progression requirement for the establishment of silencing. We observe a specific displacement of H2A.Z from chromatin in telophase-blocked cells, regardless of the silencing status of the reporter gene. These results suggest that the requirement for M-phase in the establishment of silencing may reflect a cell-cycle regulated relaxation of heterochromatin barriers.;Inactivation of Scc1, a cohesin complex subunit, also eliminates the cell cycle progression requirement for the establishment of silencing, raising the possibility that that H2A.Z and Scc1 have related functions. Scc1 is lost from chromosomes during anaphase, coincident with when we observe dissociation of H2A.Z from chromosomes. Several prior studies reported that H2A.Z deficient cells are defective in chromosome transmission, but the molecular nature of this defect has not been determined. We find that depletion of H2A.Z causes precocious loss of sister chromatid cohesion, without loss of Scc1 from chromosomes. H2A.Z is deposited into chromatin by the SWR1 complex, and is subject to acetylation of its four N-terminal tail lysine residues by the NuA4 and SAGA histone acetyltransferase complexes. We found that cells compromised for function of the SWR1 complex were defective in cohesion, as were cells expressing a form of H2A.Z not subject to acetylation. Finally, inactivation of H2A.Z in metaphase-blocked cells led immediately to cohesion defects, suggesting a direct role for H2A.Z in maintaining sister chromatid cohesion.