Pif1 Regulates Telomere Length by Removing Telomerase from Telomere Ends

Abstract

The repetitive TG-rich DNA sequence at the chromosome end, telomere, protects cells from the end-replication problem and abnormal DNA degradation in eukaryotic cells. The maintenance of telomere length is responsible for aging as well as cancer cell proliferation. Saccharomyces cerevisiae telomerase (Est2) is the reverse transcriptase responsible for extending telomeres in yeast. Pif1 helicase has been implicated in regulating the telomerase activity. We used single-molecule tethered particle motion experiments to directly investigate how Pif1 helicases regulate telomerase activity. We found that Est2 telomerase stayed bound to the telomere end after extension, but Pif1 helicases remove telomerase from the telomere. In the presence of Pif1 helicases, multiple runs of the telomerase-mediated telomere lengthening were observed. This suggests a model that Pif1 helicases remove telomerase from the telomere ends, allowing the telomerase recycling. We also observed that the ssDNA gap size affects the telomerase removal efficiency by Pif1 helicases, but not for the helicase unwinding efficiency. It is likely that efficient removal of telomerase from the telomere end is favored by the presence of multiple encountering of Pif1 helicases.