FIG. 1.
(A) A screen for second-site suppressors of the Sir3 A2G silencing defect. Expected results for wild-type Sir3, the Sir3 A2G mutant, and a putative suppressor (sup X) of Sir3 A2G are depicted. Strain XRY36 was used for the screen. 5-FOAr, 5-FOA resistant. (B) H3 D77N and H4 H75Y mutants can suppress the silencing defects of some Sir3 Ala2 mutants. MATα sir3Δ sir1Δ strains carrying the indicated alleles of Sir3 Ala2 in combination with either a plasmid encoding wild-type (WT) H3/H4 (top panels) or H3 D77N/H4 (middle panels) or H3/H4 H75Y (bottom panels) were assayed for silencing at HML and HMR, using strains VSY29, VSY30, and VSY31, respectively. Silencing at HML::URA3 was assessed on 5-FOA medium, and silencing at HMR by a mating assay. Silencing at a telomere was assessed by pink colony color on low-adenine medium.