In the absence of telomerase, telomeres progressively shorten with every round of DNA replication, leading to replicative senescence. In telomerase-deficient Saccharomyces cerevisiae, the shortest telomere triggers the onset of senescence by activating the DNA damage checkpoint and recruiting homologous recombination (HR) factors. Yet, the molecular structures that trigger this checkpoint and the mechanisms of repair have remained elusive. By tracking individual telomeres, we show that telomeres are subjected to different pathways depending on their length. We first demonstrate a progressive accumulation of subtelomeric single-stranded DNA (ssDNA) through 5′-3′ resection as telomeres shorten. Thus, exposure of subtelomeric ssDNA could be the signal for cell cycle arrest in senescence. Strikingly, early after loss of telomerase, HR counteracts subtelomeric ssDNA accumulation rather than elongates telomeres. We then asked whether replication repair pathways contribute to this mechanism. We uncovered that Rad5, a DNA helicase/Ubiquitin ligase of the error-free branch of the DNA damage tolerance (DDT) pathway, associates with native telomeres and cooperates with HR in senescent cells. We propose that DDT acts in a length-independent manner, whereas an HR-based repair using the sister chromatid as a template buffers precocious 5′-3′ resection at the shortest telomeres.

In eukaryotes, telomeres cap chromosome ends to maintain genomic stability. Failure to maintain telomeres leads to their progressive erosion and eventually triggers replicative senescence, a pathway that protects against unrestricted cell proliferation. However, the mechanisms underlying the variability and dynamics of this pathway are still elusive. Here we use a microfluidics-based live-cell imaging assay to investigate replicative senescence in individual Saccharomyces cerevisiae cell lineages following telomerase inactivation. We characterize two mechanistically distinct routes to senescence. Most lineages undergo an abrupt and irreversible switch from a replicative to an arrested state, consistent with telomeres reaching a critically short length. In contrast, other lineages experience frequent and stochastic reversible arrests, consistent with the repair of accidental telomere damage by Pol32, a subunit of polymerase δ required for break-induced replication and for post-senescence survival. Thus, at the single-cell level, replicative senescence comprises both deterministic cell fates and chaotic cell division dynamics.

Passage of the replication fork through telomeric repeats necessitates additional DNA processing by DNA repair factors, to regenerate the terminal 3’-overhang structure at leading telomeres. These factors are prevented from promoting telomeric recombination or fusion by an uncharacterized mechanism. Here we show that Rad5, a DNA helicase and ubiquitin ligase involved in the DNA damage tolerance pathway, participates in this mechanism. Rad5 is enriched at telomeres during telomere replication. Accelerated senescence seen in the absence of telomerase and Rad5, can be compensated for by a pathway involving the Rad51 recombinase and counteracted by the helicase Srs2. However, this pathway is only active at short telomeres. Instead, the ubiquitous activity of Rad5 during telomere replication is necessary for the proper reconstitution of the telomeric 3’-overhang, indicating that Rad5 is required to coordinate telomere maturation during telomere replication.