SUM1-1, a dominant suppressor of SIR mutations in Saccharomyces cerevisiae, increases transcriptional silencing at telomeres and HM mating-type loci and decreases chromosome stability.
Transcriptional silencing in the yeast Saccharomyces cerevisiae occurs at HML and HMR mating-type loci and telomeres and requires the products of the silent information regulator (SIR) genes. Recent evidence suggests that the silencer- and telomere-binding protein Rap1p initiates silencing by recruiting a complex of Sir proteins to the chromosome, where they act in some way to modify chromatin structure or accessibility. A single allele of the SUM1gene (SUM1-1) which restores silencing at HM loci in strains mutant for any of the four SIR genes was identified a number of years ago. However, conflicting genetic results and the lack of other alleles of SUM1 made it difficult to surmise the wild-type function of SUM1 or the manner in which the SUM1-1 mutation restores silencing in sir mutant strains. Here we report the cloning and characterization of the SUM1 gene and the SUM1-1 mutant allele. Our results indicate that SUM1-1 is an unusual altered-function mutation that can bypass the need for SIR function in HM silencing and increase repression at telomeres. A sum1 deletion mutation has only minor effects on silencing in SIR strains and does not restore silencing in sir mutants. In addition to its effect on transcriptional silencing, the SUM1-1 mutation (but not a sum1 deletion) increases the rate of chromosome loss and cell death. We suggest several speculative models for the action of SUM1-1 in silencing based on these and other data.
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Department of Microbiology, College of Physicians and Surgeons of Columbia University, New York, New York 10032, USA.
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D000595:Amino Acid Sequence; D001483:Base Sequence; D002875:Chromosomes / Q000648:ultrastructure; D003001:Cloning, Molecular; D005656:Fungal Proteins / Q000235:genetics*; D015966:Gene Expression Regulation, Fungal; D005799:Genes, Dominant; D005800:Genes, Fungal; D016153:Genes, Suppressor; D008565:Membrane Proteins / Q000502:physiology*; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics* / Q000502:physiology*; D012333:RNA, Messenger / Q000235:genetics; D012097:Repressor Proteins; D015183:Restriction Mapping; D012441:Saccharomyces cerevisiae / Q000235:genetics*; D029701:Saccharomyces cerevisiae Proteins; D016615:Telomere / Q000502:physiology*; D014158:Transcription, Genetic
D005656:Fungal Proteins; D008565:Membrane Proteins; D009687:Nuclear Proteins; D012333:RNA, Messenger; D012097:Repressor Proteins; C100925:SUM1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins