Tethered Sir3p Nucleates Silencing at Telomeres and Internal Loci in Saccharomyces cerevisiae

Rap1p binds to sites embedded within the Saccharomyces cerevisiae telomeric TG1-3 tract. Previous studies have led to the hypothesis that Rap1p may recruit Sir3p and Sir3p-associating factors to the telomere. To test this, we tethered Sir3p adjacent to the telomere via LexA binding sites in the rap1-17 mutant that truncates the Rap1p C-terminal 165 amino acids thought to contain sites for Sir3p association. Tethering of LexA-Sir3p adjacent to the telomere is sufficient to restore telomeric silencing, indicating that Sir3p can nucleate silencing at the telomere. Tethering of LexA-Sir3p or the LexA-Sir3p(N2O5) gain-of-function protein to a telomeric LexA site hyperrepresses an adjacent ADE2 gene in wild-type cells. Hence, Sir3p recruitment to the telomere is limiting in telomeric silencing. In addition, LexA-Sir3p(N2O5) hyperrepresses telomeric silencing when tethered to a subtelomeric site 3.6 kb from the telomeric tract. This hyperrepression is dependent on the C terminus of Rap1p, suggesting that subtelomeric LexA-Sir3p(N205) can interact with Rap1p-associated factors at the telomere. We also demonstrate that LexA-Sir3p or LexA-Sir3p(N205) tethered in cis with a short tract of telomeric TG1-3 sequences is sufficient to confer silencing at an internal chromosomal position. Internal silencing is enhanced in rap1-17 strains. We propose that sequestration of silencing factors at the telomere limits the efficiency of internal silencing.