A unique class of conditional sir2 mutants displays distinct silencing defects in Saccharomyces cerevisiae

Silencing provides a critical means of repressing transcription through the assembly and modification of chromatin proteins. The NAD(+)-dependent deacetylation of histones by the Sir2p family of proteins lends mechanistic insight into how SIR2 contributes to silencing. Here we describe three locus-specific sir2 mutants that have a spectrum of silencing phenotypes in yeast. These mutants are dependent on SIR1 for silencing function at the HM silent mating-type loci, display distinct phenotypes at the rDNA, and have dominant silencing defects at the telomeres. Telomeric silencing is restored if the mutant proteins are directly tethered to subtelomeric regions, via a Gal4p DNA-binding domain (GBD), or are recruited by tethered GBD-Sir1p. These sir2 mutations are found within conserved residues of the SIR2 family and lead to defects in catalytic activity. Since one of the mutations lies outside the previously defined minimal catalytic core, our results show that additional regions of Sir2p can be important for enzymatic activity and that differences in levels of activity may have distinct effects at the silenced loci.