Genetic analysis of Rap1p/Sir3p interactions in telomeric and HML silencing in Saccharomyces cerevisiae

Abstract

We have identified three SIR3 suppressors of the telomeric silencing defects conferred by missense mutations within the Rap1p C-terminal tail domain (aa800-827). Each SIR3 suppressor was also capable of suppressing a ra1p allele (rap1-21), which deletes the 28 aa C-terminal allele domaine, but none of suppressors restored telomeric silencing to a 165 amino acid truncation allele. These data suggest a Ra1p site for Sir3p association between the two truncation points (aa 664-799). In SIR3 suppressor strains lacking the Rap1p C-terminal tail domain, the presence of a second intragenic mutation within the rap1s domain (aa 727-747), enhanced silencing 30-300 fold. These data suggest a competition between Sir3p and factors that interfere with silencing for association in the rap1 domain, rap1-21 strains containing both wild-type Sir3p and either of the Sir3 suppressor proteins displayed a 400-4000-fold increase in telomeric silencing over rap1-21 strains carrying either Sir3p suppressor in the absence of wild-type Sir3p. We propose that this telomere-specific synergism is medicated in part through stabilization of Rap1p/Sir3p telomeric complexes by Sir3p-Sir3p interactions.