Ordered nucleation and spreading of silenced chromatin in XXXX .

@entity13442 @entity34707 @entity375 @entity33739
In @entity375 , 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, @entity34707 , and @entity13442 were spread throughout and coincident with the silenced region at HMR. @entity33739 , in contrast, associated only with the HMR-E silencer, consistent with its role in establishment but not maintenance of silencing. @entity13442 was required for the association of other Sir proteins with silencers. In contrast, in the absence of Sir2p or @entity34707 , partial assemblies of Sir proteins could form at silencers, where Sir protein assembly began. Spreading across HMR required Sir2p and @entity34707 , 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, @entity34707 , and @entity13442 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.
