Identification of a novel allele of SIR3 defective in the maintenance, butnot the establishment, of silencing in Saccharomyces cerevisiae

Using a screen for genes that affect telomere function, we isolated sir3-P8 98R, an allele of SIR3 that reduces telomeric silencing yet does not affect mating. While sir3-P898R mutations cause no detectable mating defect in qu antitative assays, they result in synergistic mating defects in combination with mutations such as sir1 that affect the establishment of silencing. In contrast, sir3-P898R in combination with a cac1 mutation, which affects th e maintenance of silencing, does not result in synergistic mating defects. MATa sir3-P898R mutants form shmoo clusters in response to alpha-factor, an d sir3-P898R strains are capable of establishing silencing at a previously derepressed HML locus with kinetics like that of wild-type SIR3 strains. Th ese results imply that Sir3-P898Rp is defective in the maintenance, but not the establishment of silencing. In addition, overexpression of a C-termina l fragment of Sir3-P898R results in a dominant nonmating phenotype: HM sile ncing is completely lost at both HML and HMR. Furthermore, HM silencing is most vulnerable to disruption by the Sir3-P898R C terminus immediately afte r S-phase, the time when ne iv silent chromatin is assembled onto newly rep licated DNA.