EXO1 Contributes to Telomere Maintenance in Both Telomerase-Proficient and Telomerase-Deficient Saccharomyces cerevisiae

摘要

Previous work in budding yeast has indicated that telomeres are protected, at least in part, from the action of Exo1, which degrades the C-rich strand of partially uncapped telomeres. To explore this further, we examined the consequences of Exo1-mediated activity in strains that lacked Ku, telomerase, or both. Loss of Exo1 partially rescued the telomere length defect in a yku80 Δ strain, demonstrating that exonuclease action can directly contribute to telomere shortening. The rapid loss of inviability displayed by a yku80 Δ est2 Δ strain was also partially alleviated by an exo1 Δ mutation, further supporting the proposal that Exo1 is one target of the activities that normally protect wild-type telomeres. Conversely, however, Exo1 activity was also capable of enhancing telomere function and consequently cell proliferation, by contributing to a telomerase-independent pathway for telomere maintenance. The recovery of recombination-dependent survivors that arose in a yku80 Δ est2 Δ strain was partially dependent on Exo1 activity. Furthermore, the types of recombination events that facilitate telomerase-independent survival were influenced by Exo1 activity, in both est2 Δ and yku80 Δ est2 Δ strains. These data demonstrate that Exo1 can make either positive or negative contributions to telomere function and cell viability, depending on whether telomerase or recombination is utilized to maintain telomere function.