Telomerase, the recombination machinery and Rap1 play redundant roles in yeast telomere protection.

Telomeres are specialized nucleoprotein complexes that protect the ends of eukaryotic chromosomes and distinguish them from broken DNA ends. Disruption of telomere protection may cause aging-associated pathologies and cancer. Here, we examined what makes telomere protection durable and resistant to perturbations using a budding yeast model organism. The protein Rap1 binds the telomeric repeats, negatively regulates telomere length, and protects telomeres by repressing homologous recombination and non-homologous end joining (NHEJ). A single-nucleotide mutation in the Kluyveromyces lactis telomerase RNA (TER1) template, ter1-16T, is incorporated into the telomeric repeats, disrupting the binding of Rap1 and causing dramatic telomere elongation. However, cell viability is not significantly affected, suggesting the existence of additional mechanism(s) for telomere protection. To examine this hypothesis, we explored the contribution of the recombination factor Rad52 and telomerase to telomere protection in the background of ter1-16T. To disrupt the function of telomerase, we exploited small mutations in a stem-loop domain of TER1 (Reg2), which result in short but stable telomeres. We generated K. lactis strains with combinations of three different mutations: ter1-16T, RAD52 deletion, and a two-nucleotide substitution in Reg2. Our results show that upon Rap1 depletion from telomeres, telomerase and the recombination machinery compensate for the loss of Rap1 protection and play redundant but critical roles in preventing NHEJ and maintaining telomere integrity and cell viability. These results demonstrate how redundant pathways make the essential role of telomeres-protecting our genome integrity and preventing cancer-more robust and resistant to assaults and perturbations.