Hir Proteins Are Required for Position-Dependent Gene Silencing in Saccharomyces cerevisiae in the Absence of Chromatin Assembly Factor I

Chromatin assembly factor I (CAF-I) is a three-subunit histone-binding complex conserved from the yeastSaccharomyces cerevisiae to humans. Yeast cells lacking CAF-I (cac Δ mutants) have defects in heterochromatic gene silencing. In this study, we showed that deletion ofHIR genes, which regulate histone gene expression, synergistically reduced gene silencing at telomeres and at theHM loci incac Δ mutants, althoughhir Δ mutants had no silencing defects when CAF-I was intact. Therefore, Hir proteins are required for an alternative silencing pathway that becomes important in the absence of CAF-I. Because Hir proteins regulate expression of histone genes, we tested the effects of histone gene deletion and overexpression on telomeric silencing and found that alterations in histone H3 and H4 levels or in core histone stoichiometry reduced silencing incac Δ mutants but not in wild-type cells. We therefore propose that Hir proteins contribute to silencing indirectly via regulation of histone synthesis. However, deletion of combinations ofCAC andHIR genes also affected the growth rate and in some cases caused partial temperature sensitivity, suggesting that global aspects of chromosome function may be affected by the loss of members of both gene families.