yKu70/yKu80 and Rif1 Regulate Silencing Differentially at Telomeres in Candida glabrata

Candida glabrata , a common opportunistic fungal pathogen, adheres efficiently to mammalian epithelial cells in culture. This interaction in vitro depends mainly on the adhesin Epa1, one of a large family of cell wall proteins. Most of theEPA genes are located in subtelomeric regions, where they are transcriptionally repressed by silencing. In order to better characterize the transcriptional regulation of theEPA family, we have assessed the importance ofC. glabrata orthologues of known regulators of subtelomeric silencing inSaccharomyces cerevisiae. To this end, we used a series of strains containing insertions of the reporterURA3 gene within different intergenic regions throughout four telomeres ofC. glabrata . Using these reporter strains, we have assessed the roles ofSIR2 ,SIR3 ,SIR4 ,HDF1 (yKu70),HDF2 (yKu80), andRIF1 in mediating silencing at fourC. glabrata telomeres. We found that, whereas theSIR proteins are absolutely required for silencing of the reporter genes and the native subtelomericEPA genes, the Rif1 and the Ku proteins regulate silencing at only a subset of the analyzed telomeres. We also mapped acis element adjacent to theEPA3 locus that can silence a reporter gene when placed at a distance of 31 kb from the telomere. Our data show that silencing of theC. glabrata telomeres varies from telomere to telomere. In addition, recruitment of silencing proteins to the subtelomeres is likely, for certain telomeres, to depend both on the telomeric repeats and on particular discrete silencing elements.