The Est1 subunit of yeast telomerase binds the Tlc1 telomerase RNA

Est1 is a component of yeast telomerase, and est1 mutants have senescence a nd telomere loss phenotypes, The exact function of Est1 is not known, and i t is not homologous to components of other telomerases. We previously showe d that Est1 protein coimmunoprecipitates with Tlc1 (the telomerase RNA) as well as with telomerase activity. Est1 has homology to Ebs1, an uncharacter ized yeast open reading frame product, including homology to a putative RNA recognition motif (RRM) of Ebs1, Deletion of EBS1 results in short telomer es. We created point mutations in a putative RRM of Est1. One mutant was un able to complement either the senescence or the telomere loss phenotype of est1 mutants. Furthermore, the mutant protein no longer coprecipitated with the Tlc1 telomerase RNA. Mutants defective in the binding of Tlc1 RNA were nevertheless capable of binding single-stranded TG-rich DNA. Our data sugg est that an important role of Est1 in the telomerase complex is to bind to the Tlc1 telomerase RNA via an RRM. Since Est1 can also bind telomeric DNA, Est1 may tether telomerase to the telomere.