SUPPRESSORS OF DEFECTIVE SILENCING IN YEAST - EFFECTS ON TRANSCRIPTIONAL REPRESSION AT THE HMR LOCUS, CELL-GROWTH AND TELOMERE STRUCTURE

To identify factors that affect transcriptional silencing at the HMR m ating-type locus in yeast, we characterized a set of extragenic suppre ssor mutations that restore metastable repression in cells containing both a mutant silencer-binding protein (rap1(s)) and a mutated silence r element (hmr Delta A). A total of 57 suppressors comprising 21 diffe rent complementation groups was identified. This report describes a de tailed genetic analysis of these suppressors of defective silencing (s ds) mutants. The sds mutants fall into several distinct categories bas ed on secondary phenotypes, such as their ability to suppress the rap1 (s) telomere lengthening phenotype, general effects on telomere length , temperature-dependent growth defects, and the ability to bypass the requirement for cis regulatory elements at the HMR-E silencer. One par ticular mutant, sds4-1, strongly suppresses the rap1(s) silencing defe ct, restores telomeres to nearly wild-type length, and displays a seve re growth defect at all temperatures. SDS4 mutations also suppress the silencing defect caused by mutations in the RAP1-interacting factor R IF1. We cloned the SDS4 gene and show that it is identical to GAL11(SP T13), which encodes a component of a protein complex that mediates tra nscriptional activation. Possible mechanism(s) of suppression by sds4 and the other sds mutations is discussed.