Ku-deficient yeast strains exhibit alternative states of silencing competence

In Saccharomyces cerevisiae, efficient silencer function requires telomere proximity, i.e. compartments of the nucleoplasm enriched in silencing facto rs. Accordingly, silencers located far from telomeres function inefficientl y. We show here that cells lacking yKu balance between two mitotically stab le states of silencing competence. In one, a partial delocalization of telo meres and silencing factors throughout the nucleoplasm correlates with enha nced silencing at a non-telomeric locus, while in the other, telomeres reta in their focal pattern of distribution and there is no repression at the no n-telomeric locus, as observed in wild-type cells. The two states also diff er in their level of residual telomeric silencing. These findings indicate the existence of a yKu-independent pathway of telomere clustering and Sir l ocalization. Interestingly, this pathway appears to be under epigenetic con trol.