Genetic Analysis of Rap1p/Sir3p Interactions in Telomeric and Hml Silencing in Saccharomyces Cerevisiae

We have identified three SIR3 suppressors of the telomeric silencing defects conferred by missense mutations within the Rap1p C-terminal tail domain (aa 800-827). Each SIR3 suppressor was also capable of suppressing a rap1 allele (rap1-21), which deletes the 28 aa C-terminal tail domain, but none of the suppressors restored telomeric silencing to a 165 amino acid truncation allele. These data suggest a Rap1p 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(s) 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 mediated in part through stabilization of Rap1p/Sir3p telomeric complexes by Sir3p-Sir3p interactions.