title: Structural analyses of Sum1-1p-dependent transcriptionally silent chromatin in Saccharomyces cerevisiae.
abstract: In Saccharomyces cerevisiae, transcriptional silencing of the cryptic mating loci HML and HMR is established by the combined actions of cis-acting silencers and trans-acting proteins, including Sir2p, Sir3p and Sir4p. The Sir proteins serve as an integral part of a special silent chromatin at the HM loci. Deletion of any of the SIR2-SIR4 genes leads to a complete loss of silencing. However, the SUM1-1 mutation can restore silencing at the HM loci. Recently, it has been shown that Sum1-1p is directed to the silencers and internal regions of the HM loci, and interacts with the Hst1p histone deacetylase that is a paralog of the Sir2p histone deacetylase. Like Sir-dependent silent chromatin, Sum1-1p-dependent chromatin is hypoacetylated. These suggest that Sum1-1p and Hst1p play roles similar to those of the Sir proteins in promoting transcriptional silencing. Here, we examine whether Sum1-1p-dependent chromatin is similar to Sir-dependent silent chromatin, which is characterized by densely and precisely positioned nucleosomes. We demonstrate that Sum1-1p-dependent primary chromatin structure at HMR largely resembles, but is not identical with, Sir-dependent silent chromatin, whereas Sum1-1p-dependent HML chromatin largely resembles, but is not identical with, derepressed chromatin found in a sir- background. This correlates with the previous finding that SUM1-1 restores silencing more efficiently at HMR than at HML. We show also that DNA in Sum1-1p-dependent silent chromatin assumes a distinct topology. Moreover, we present evidence indicating that Sum1-1p can increase the stability of Sir-dependent silent chromatin, thereby providing an explanation for the finding that SUM1-1 enhances HML/HMR silencing in a SIR+ background.
authors.affiliation: Department of Biology University of Rochester Rochester, NY 14627, USA.
authors.forename: Qun
authors.initials: Q
authors.lastname: Yu
authors.forename: Susan
authors.initials: S
authors.lastname: Elizondo
authors.forename: Xin
authors.initials: X
authors.lastname: Bi
mesh_terms: D002843:Chromatin; D004247:DNA / Q000737:chemistry; D020868:Gene Silencing; D006655:Histone Deacetylases / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics / Q000378:metabolism*; D009690:Nucleic Acid Conformation; D012097:Repressor Proteins; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism*; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D056565:Sirtuin 2; D037761:Sirtuins / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D002843:Chromatin; D009687:Nuclear Proteins; D012097:Repressor Proteins; C084122:SIR3 protein, S cerevisiae; C061573:SIR4 protein, S cerevisiae; C100925:SUM1 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D004247:DNA; C087778:SIR2 protein, S cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D006655:Histone Deacetylases