THEME AND VARIATION AMONG SILENCING PROTEINS IN SACCHAROMYCES-CEREVISIAE AND KLUYVEROMYCES-LACTIS

The cryptic mating type loci in Saccharomyces cerevisiae act as reserv oirs of mating type information used in mating type switching in homot hallic yeast strains. The transcriptional silencing of these loci depe nds on the formation of a repressive chromatin structure that is remin iscent of heterochromatin. Silent information regulator (Sir) proteins 2-4 are absolutely required for silencing. To learn more about silenc ing, we investigated mating type and Sir proteins in the yeast Kluyver omyces lactis, which contains cryptic copies of the mating type genes. A functional homolog of SIR4 from K. lactis complements the silencing defect of sir4 null mutations in S. cerevisiae. K. lactis sir2 and si r4 mutant strains showed partial derepression of the silent oil gene, establishing that the silencing role of these proteins is conserved. K . lactis sir2 mutants are more sensitive than the wild type to ethidiu m bromide, and K. lactis sir4 mutants are more resistant phenotypes th at are not observed for the corresponding mutants of S. cerevisiae. Fi nally, the deletion of sir4 in the two yeasts leads to opposite effect s on telomere length. Thus, Sir proteins from K. lactis have roles in both silencing and telomere length maintenance, reflecting conserved f unctional themes. The various phenotypes of sir mutants in K. lactis a nd S. cerevisiae, however, revealed unanticipated variation between th eir precise roles.