SummaryRap1 is the main protein that binds double-stranded telomeric DNA in Saccharomyces cerevisiae. Rap1 can also bind and promote the formation of G-quadruplexes, which are thought to form at telomeres. Examination of the telomere functions of Rap1 is complicated by the fact that it also acts as a transcriptional regulator of hundreds of genes and is encoded by an essential gene. In this study, we disrupt Rap1 telomere association and G-quadruplex formation by expressing a mutant telomerase RNA subunit (tlc1-tm) that introduces mutant telomeric repeats. Remarkably, tlc1-tm cells grow as well as wild-type cells, although depletion of Rap1 at telomeres causes defects in telomere length regulation and telomere capping. We find that Rap1, Rif2, and the Ku complex work in parallel to prevent telomere degradation, and absence of all three causes lethality. The partially redundant mechanisms may explain the rapid evolution of telomere components in budding yeast species.
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