title: SAS4 and SAS5 are locus-specific regulators of silencing in Saccharomyces cerevisiae.
abstract: Sir2p, Sir3p, Sir4p, and the core histones form a repressive chromatin structure that silences transcription in the regions near telomeres and at the HML and HMR cryptic mating-type loci in Saccharomyces cerevisiae. Null alleles of SAS4 and SAS5 suppress silencing defects at HMR; therefore, SAS4 and SAS5 are negative regulators of silencing at HMR. This study revealed that SAS4 and SAS5 contribute to silencing at HML and the telomeres, indicating that SAS4 and SAS5 are positive regulators of silencing at these loci. These paradoxical locus-specific phenotypes are shared with null alleles of SAS2 and are unique among phenotypes of mutations in other known regulators of silencing. This work also determined that these SAS genes play roles that are redundant with SIR1 at HML, yet distinct from SIR1 at HMR. Furthermore, these SAS genes are not redundant with each other in silencing HML. Collectively, these data suggest that SAS2, SAS4, and SAS5 constitute a novel class of regulators of silencing and reveal fundamental differences in the regulation of silencing at HML and HMR. We provide evidence for a model that accounts for the observation that these SAS genes are both positive and negative regulators of silencing.
authors.affiliation: Department of Cell and Structural Biology and Department of Microbiology, University of Illinois, Urbana, Illinois 61801, USA.
authors.forename: E Y
authors.initials: EY
authors.lastname: Xu
authors.forename: S
authors.initials: S
authors.lastname: Kim
authors.forename: D H
authors.initials: DH
authors.lastname: Rivier
mesh_terms: D000483:Alleles; D002843:Chromatin / Q000235:genetics / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D017353:Gene Deletion; D018628:Gene Dosage; D015966:Gene Expression Regulation, Fungal; D005800:Genes, Fungal; D049770:Genes, Mating Type, Fungal; D008957:Models, Genetic; D010641:Phenotype; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D020218:Response Elements / Q000235:genetics*; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000502:physiology; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D013489:Suppression, Genetic; D016615:Telomere / Q000235:genetics; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism
chemical_list: D002843:Chromatin; D005656:Fungal Proteins; D012097:Repressor Proteins; C084121:SIR1 protein, S cerevisiae; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D015534:Trans-Activators