title: Mechanism for epigenetic variegation of gene expression at yeast telomeric heterochromatin.
abstract: Yeast contains heterochromatin at telomeres and the silent mating-type loci (HML/HMR). Genes positioned within the telomeric heterochromatin of Saccharomyces cerevisiae switch stochastically between epigenetically bistable ON and OFF expression states. Important aspects of the mechanism of variegated gene expression, including the chromatin structure of the natural ON state and the mechanism by which it is maintained, are unknown. To address this issue, we developed approaches to select cells in the ON and OFF states. We found by chromatin immunoprecipitation (ChIP) that natural ON telomeres are associated with Rap1 binding and, surprisingly, also contain known characteristics of OFF telomeres, including significant amounts of Sir3 and H4K16 deacetylated nucleosomes. Moreover, we found that H3K79 methylation (H3K79me), H3K4me, and H3K36me, which are depleted from OFF telomeres, are enriched at ON telomeres. We demonstrate in vitro that H3K79me, but not H3K4me or H3K36me, disrupts transcriptional silencing. Importantly, H3K79me does not significantly reduce Sir complex binding in vivo or in vitro. Finally, we show that maintenance of H3K79me at ON telomeres is dependent on transcription. Therefore, although Sir proteins are required for silencing, we propose that epigenetic variegation of telomeric gene expression is due to the bistable enrichment/depletion of H3K79me and not the fluctuation in the amount of Sir protein binding to nucleosomes.
authors.affiliation: Department of Biological Chemistry, David Geffen School of Medicine, University of California at Los Angeles, Los Angeles, CA 90095, USA.
authors.forename: Tasuku
authors.initials: T
authors.lastname: Kitada
authors.forename: Benjamin G
authors.initials: BG
authors.lastname: Kuryan
authors.forename: Nancy Nga Huynh
authors.initials: NN
authors.lastname: Tran
authors.forename: Chunying
authors.initials: C
authors.lastname: Song
authors.forename: Yong
authors.initials: Y
authors.lastname: Xue
authors.forename: Michael
authors.initials: M
authors.lastname: Carey
authors.forename: Michael
authors.initials: M
authors.lastname: Grunstein
mesh_terms: D003470:Culture Media; D019175:DNA Methylation; D057890:Epigenomics; D005434:Flow Cytometry; D015966:Gene Expression Regulation, Fungal; D020868:Gene Silencing; D014644:Genetic Variation; D006570:Heterochromatin / Q000235:genetics*; D006657:Histones / Q000378:metabolism; D011485:Protein Binding; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D016615:Telomere / Q000235:genetics*
chemical_list: D003470:Culture Media; D006570:Heterochromatin; D006657:Histones; D029701:Saccharomyces cerevisiae Proteins