text: SIR3 is principally involved in heterochromatin spreading, the silencing activity of the SIR protein complex. When overexpressed, SIR3 leads to spreading beyond the normal nucleation site. SIR3 can continue to operate at very low levels of SIR2 and SIR4, but not without them. It preferentially binds to unmodified nucleosomes (no acetylation at H4K16 or methylation at H3K79), and relies on SIR2's deacetylation of H4K16 to enhance silencing. H3K79 methylation by DOT1 methyltransferase inhibits SIR3, resulting in an unsilenced chromatin region. SIR3 is recruited to target sequence by the transcription factors RAP1 or ABF1.