Figure 1.

Molecular model for silencing at HM mating-type loci and telomeres. Silencing is initiated by DNA-binding proteins [Rap1p and origin recognition complex (ORC), or Rap1p and Ku] that cooperate to recruit SIR protein complexes. These histone-interacting SIR complexes then assemble along adjacent nucleosomal DNA, masking or in some other way altering the nearby chromatin. At telomeres, where silencing is unstable, or variegated, recruitment of SIRs is negatively regulated by the RIF proteins