The Ku Complex in Silencing the Cryptic Mating-Type Loci ofSaccharomyces cerevisiae

Sir1 establishes transcriptional silencing at the cryptic mating-type loci HMR and HML (HM loci) by recruiting the three other Sir proteins, Sir2, -3, and -4, that function directly in silenced chromatin. However, SIR1-independent mechanisms also contribute to recruiting the Sir2-4 proteins to the HM loci. A screen to elucidate SIR1-independent mechanisms that establish HMR silencing identified a mutation in YKU80. The role for Ku in silencing both HMR and HML was masked by SIR1. Ku's role in silencing the HM loci was distinct from its shared role with the nuclear architecture protein Esc1 in tethering the HM loci and telomeres to the nuclear periphery. The ability of high-copy SIR4 to rescue HMR silencing defects in sir1Delta cells required Ku, and chromatin immunoprecipitation (ChIP) experiments provided evidence that Ku contributed to Sir4's physical association with the HM loci in vivo. Additional ChIP experiments provided evidence that Ku functioned directly at the HM loci. Thus Ku and Sir1 had overlapping roles in silencing the HM loci.