Spreading of transcriptional repressor SIR3 from telomeric heterochromatin

TELOMERIC genes and the HM loci in Saccharomyces cerevisiae are transcriptionally repressed and adopt a heterochromatin-like structure1,2. The tram-acting factors RAP1, SIR3 and SIR4 are required for telomeric and HM silencing3–5, and are thought to be chromosomal6–8, but how they contribute to histone-dependent repression of adjacent chroma tin9,11 is unclear. SIR3 suppresses silencing defects in hi stones10, is limiting for silencing adjacent to telomeres12, and interacts with the H3 and H4 amino termini in vitro13. Here we show that SIR3 co-immunoprecipitates SIR4, RAP1 and histones from cellular extracts, suggesting the presence of large chromatin-associated protein complexes. Crosslinking experiments show that SIR3 is present at HMRa, HMLα and telomeres in vivo, and that it spreads from telomeric regions into adjacent chromatin when overexpressed. Thus SIR3 is a structural component of yeast heterochromatin, repressing adjacent genes as it spreads along the chromosome.