Ordered nucleation and spreading of silenced chromatin in Saccharomyces cerevisiae.
In Saccharomyces cerevisiae, silencing at the HM loci depends on Sir proteins, which are structural components of silenced chromatin. To explore the structure and assembly of silenced chromatin, the associations of Sir proteins with sequences across the HMR locus were examined by chromatin immunoprecipitation. In wild-type cells, Sir2p, Sir3p, and Sir4p were spread throughout and coincident with the silenced region at HMR. Sir1p, in contrast, associated only with the HMR-E silencer, consistent with its role in establishment but not maintenance of silencing. Sir4p was required for the association of other Sir proteins with silencers. In contrast, in the absence of Sir2p or Sir3p, partial assemblies of Sir proteins could form at silencers, where Sir protein assembly began. Spreading across HMR required Sir2p and Sir3p, as well as the deacetylase activity of Sir2p. These data support a model for the spreading of silenced chromatin involving cycles of nucleosome deacetylation by Sir2p followed by recruitment of additional Sir2p, Sir3p, and Sir4p to the newly deacetylated nucleosome. This model suggests mechanisms for boundary formation, and for maintenance and inheritance of silenced chromatin. The principles are generalizable to other types of heritable chromatin states.
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720, USA.
Laura N
LN
Rusché
Ann L
AL
Kirchmaier
Jasper
J
Rine
D000818:Animals; D002843:Chromatin / Q000235:genetics* / Q000378:metabolism; D015825:Chromosomes, Fungal; D004268:DNA-Binding Proteins / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D020868:Gene Silencing; D005800:Genes, Fungal; D006655:Histone Deacetylases / Q000378:metabolism; D006657:Histones / Q000235:genetics / Q000378:metabolism; D046911:Macromolecular Substances; D008957:Models, Genetic; D009687:Nuclear Proteins / Q000378:metabolism; D009707:Nucleosomes / Q000235:genetics / Q000378:metabolism; D011233:Precipitin Tests; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000502:physiology; D037941:Silencer Elements, Transcriptional / Q000235:genetics; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae / Q000378:metabolism*; D037761:Sirtuins / Q000378:metabolism*
D002843:Chromatin; D004268:DNA-Binding Proteins; D006657:Histones; D046911:Macromolecular Substances; D009687:Nuclear Proteins; D009707:Nucleosomes; D011993:Recombinant Fusion Proteins; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; D037761:Sirtuins; D006655:Histone Deacetylases