Barrier function at HMR.
The silenced HMR domain is restricted from spreading by barrier elements, and the right barrier is a unique t-RNA(THR) gene. We show that sequences immediately flanking the silenced domain were enriched in acetylated, but not methylated, histones, whereas the barrier element was associated with a nucleosome-free region. Surprisingly, the SAGA acetyltransferase resided across the entire region. We further demonstrate that a mutation in the barrier eliminated the nucleosome-free gap but only subtly altered the distribution of SAGA. Interestingly, neither reformation of the nucleosome nor mutations in chromatin-modifying enzymes alone led to an unrestricted spread of silenced chromatin. Double mutations in the t-RNA barrier and these complexes, on the other hand, led to a significant spread of Sir proteins. These results suggest two overlapping mechanisms function to restrict the spread of silencing: one of which involves a DNA binding element, whereas the other mechanism involves specific chromatin-modifying activities.
Oki
Masaya
M
Unit on Chromatin and Transcription, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
Kamakaka
Rohinton T
RT
D002843:Chromatin / Q000502:physiology; D042002:Chromatin Assembly and Disassembly / Q000502:physiology; D015966:Gene Expression Regulation, Fungal / Q000502:physiology; D020868:Gene Silencing / Q000502:physiology*; D006657:Histones / Q000502:physiology*; D009707:Nucleosomes / Q000502:physiology; D012363:RNA, Transfer, Thr / Q000235:genetics* / Q000502:physiology; D012441:Saccharomyces cerevisiae / Q000235:genetics; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae / Q000502:physiology
D002843:Chromatin; D006657:Histones; D009707:Nucleosomes; D012363:RNA, Transfer, Thr; C084122:SIR3 protein, S cerevisiae; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae