Dynamics of Sir3 spreading in budding yeast: secondary recruitment sites and euchromatic localization

Chromatin domains are believed to spread via a polymerization-like mechanism in which modification of a given nucleosome recruits a modifying complex, which can then modify the next nucleosome in the polymer. In this study, we carry out genome-wide mapping of the Sir3 component of the Sir silencing complex in budding yeast during a time course of establishment of heterochromatin. Sir3 localization patterns do not support a straightforward model for nucleation and polymerization, instead showing strong but spatially delimited binding to silencers, and weaker and more variable Ume6-dependent binding to novel secondary recruitment sites at the seripauperin (PAU) genes. Genome-wide nucleosome mapping revealed that Sir binding to subtelomeric regions was associated with overpackaging of subtelomeric promoters. Sir3 also bound to a surprising number of euchromatic sites, largely at genes expressed at high levels, and was dynamically recruited to GAL genes upon galactose induction. Together, our results indicate that heterochromatin complex localization cannot simply be explained by nucleation and linear polymerization, and show that heterochromatin complexes associate with highly expressed euchromatic genes in many different organisms.