The positioning of yeast telomeres depends on SIR3, SIR4, and the integrity of the nuclear membrane.

Abstract

In the yeast Saccharomyces cerevisiae, transcriptional repression at the silent mating-type loci HML and HMR and at telomeres has many of the hallmarks of position-effect variegation in Drosophila, where the condensed higher-order structure of heterochromatin “spreads” into adjacent chromatin, inactivating nearby genes (for review, see Sandell and Zakian 1992). The yeast mating-type genes have provided a genetically tractable system to identify trans-acting factors and cis-acting sequences required for this repression mechanism (for review, see Laurenson and Rine 1992). In particular, deletion studies have identified a cis-acting silencer with a minimal size of 134 bp, which is required for the repression of transcription of the mating-type genes at HMR (Brand et al. 1987). Similar elements have been mapped at HML (Mahoney and Broach 1989). Like enhancers, these silencers function in either orientation and at variable distances from the targeted promoter. A number of trans-acting factors are implicated in the mechanism of...