Sir- and Silencer-Independent Disruption of Silencing in Saccharomyces by Sas10p

A promoter fusion library of Saccharomyces cerevisiae genes was used to exploit phenotypes associated with altered protein dosage. We identified a novel gene, SAS10, by the ability of Sas10p, when overproduced, to disrupt silencing. The predicted Sas10p was 70,200 kD and strikingly rich in charged amino acids. Sas10p was exclusively nuclear in all stages of the cell cycle. Overproduction of Sas10p caused derepression of mating type genes at both HML and HMR, as well as of URA3, TRP1, and ADE2 when inserted near a telomere or at HMR or the rDNA locus. Repressed genes not associated with silenced chromatin were unaffected. Sas10p was essential for viability, and the termination point following Sas10p depletion was as large budded cells. Remarkably, Sas10p overproduction disrupted silencing even under conditions that bypassed the requirement for Sir proteins, ORC, and Rap1p in silencing. These data implied that Sas10p function was intimately connected with the structure of silenced chromatin.