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

ABSTRACT

Chromatin assembly factor I (CAF-I) is a three-subunit histone-binding complex conserved from the yeast Saccharomyces cerevisiae to humans. Yeast cells lacking CAF-I (cacΔ mutants) have defects in heterochromatic gene silencing. In this study, we showed that deletion of HIRgenes, which regulate histone gene expression, synergistically reduced gene silencing at telomeres and at the HM loci in cacΔ mutants, although hirΔ 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 in cacΔ 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 of CAC and HIR 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.

ACKNOWLEDGMENTS

We thank members of the Kaufman and Osley laboratories and D. deBruin, R. Bacon, A. Dillin, and S. Okamura for critical reviews of the manuscript; S. Liebman and J. Berman for communication of results prior to publication; J. Boeke, J. Haber, M. Grunstein, A. Lustig, J. Rine, and F. Winston for strains and plasmids; and J. Chuang for excellent technical assistance.

This work was supported by Department of Energy funds awarded to P.D.K. and administered through the Lawrence Berkeley National Laboratory under contract DE-AC03-76SF00098 and by National Institutes of Health grants GM55712 (to P.D.K.) and GM40118 (to M.A.O.).