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 yeast Saccharomyces cerevisiae to humans. Yeast cells lacking CAF-I (cacDelta mutants) have defects in heterochromatic gene silencing. In this study, we showed that deletion of HIR genes, which regulate histone gene expression, synergistically reduced gene silencing at telomeres and at the HM loci in cacDelta mutants, although hirDelta 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 cacDelta 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.
Lawrence Berkeley National Laboratory and Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3206, USA. pdkaufman@1bl.gov
P D
PD
Kaufman
J L
JL
Cohen
M A
MA
Osley
D000595:Amino Acid Sequence; D002843:Chromatin; D056504:Chromatin Assembly Factor-1; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins / Q000235:genetics / Q000502:physiology*; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism*; D018628:Gene Dosage; D015966:Gene Expression Regulation, Fungal; D006657:Histones / Q000235:genetics; D006801:Humans; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D012091:Repetitive Sequences, Nucleic Acid; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D016615:Telomere
D002843:Chromatin; D056504:Chromatin Assembly Factor-1; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; D005656:Fungal Proteins; C078808:HIR1 protein, S cerevisiae; C078809:HIR2 protein, S cerevisiae; D006657:Histones; D009687:Nuclear Proteins; D012097:Repressor Proteins; D029701:Saccharomyces cerevisiae Proteins