Locus specificity determinants in the multifunctional yeast silencing protein Sir2

Yeast SIR2, the founding member of a conserved gene family, acts to modulat e chromatin structure in three different contexts: silent (HM) mating-type loci, telomeres and rDNA, At HM loci and telomeres, Sir2p forms a complex w ith Sir3p and Sir4p, However, Sir2p's role in rDNA silencing is Sir3/4 inde pendent, requiring instead an essential nucleolar protein, Net1p. We descri be two novel classes of SIR2 mutations specific to either HM/telomere or rD NA silencing. Despite their opposite effects, both classes of mutations clu ster in the same two regions of Sir2p, each of which borders on a conserved core domain. A surprising number of these mutations are dominant. Several rDNA silencing mutants display a Sir2p nucleolar localization defect that c orrelates with reduced Net1p binding. Although the molecular defect in HM/t elomere-specific mutants is unclear, they mimic an age-related phenotype wh ere Sir3p and Sir4p relocalize to the nucleolus. Artificial targeting can c ircumvent the silencing defect in a subset of mutants from both classes. Th ese results define distinct functional domains of Sir2p and provide evidenc e for additional Sir2p-interacting factors with locus-specific silencing fu nctions.