The role of the EST genes in yeast telomere replication

We have recently completed a large mutant screen designed to identify new mutants of Saccharomyces cerevisiae with a telomerase-like defect. From this screen; 22 mutants were identified that mapped to three genes, called EST1, EST2 and EST3, as well as a novel EST-like mutation in a fourth gene, previously identified as CDC13. Mutations in each of these genes give rise to phenotypes that are indistinguishable from those observed when TLC1, encoding the yeast telomerase RNA, is deleted. In addition, genetic analysis indicates that all four genes function in the same pathway for telomere replication as defined by TLC1, the one known component of telomerase. This indicates that these genes encode factors that are essential in vivo for telomerase function. Genetic and biochemical analyses have shown that EST1 and CDC13 encode single-stranded telomeric DNA-binding proteins, suggesting that these two proteins may function to mediate access of telomerase to the end of the telomere.