The telomeres protect chromosome ends from fusion and degradation1. In the absence of a specific telomere elongation mechanism, their DNA progressively shortens with every round of replication leading to replicative senescence2. Here, we show that telomerase-deficient cells bearing a single very short telomere senesce earlier, demonstrating that length of the shortest telomere is a major determinant of the onset of senescence. We further show that Mec1p/ATR recognises specifically the single very short telomere causing the accelerated senescence. Strikingly, before entry into senescence, cells divide for some generations despite a complete erosion of their shortened telomeres. This pre-senescence growth requires RAD52 and MMS1 and there is no evidence for major inter-telomeric recombination. We propose that, in the absence of telomerase, a very short telomere is first maintained in a pre-signalling state by a RAD52/MMS1 dependent pathway and then switches to a signalling state leading to senescence through a Mec1p-dependent checkpoint.

The ends of linear eukaryotic chromosomes are protected by telomeres, which serve to ensure proper chromosome replication and to prevent spurious recombination at chromosome ends. In this study, we show by single cell analysis that in the absence of telomerase, a single short telomere is sufficient to induce the recruitment of checkpoint and recombination proteins. Notably, a DNA damage response at eroded telomeres starts many generations before senescence and is characterized by the recruitment of Cdc13 (cell division cycle 13), replication protein A, DNA damage checkpoint proteins and the DNA repair protein Rad52 into a single focus. Moreover, we show that eroded telomeres, although remaining at the nuclear periphery, move to the nuclear pore complex. Our results link the DNA damage response at eroded telomeres to changes in subnuclear localization and suggest the existence of collapsed replication forks at eroded telomeres.

Telomerase maintains chromosome ends from humans to yeasts. Recruitment of yeast telomerase to telomeres occurs through its Ku and Est1 subunits via independent interactions with telomerase RNA (TLC1) and telomeric proteins Sir4 and Cdc13, respectively. However, the structures of the molecules comprising these telomerase-recruiting pathways remain unknown. Here, we report crystal structures of the Ku heterodimer and Est1 complexed with their key binding partners. Two major findings are as follows: (1) Ku specifically binds to telomerase RNA in a distinct, yet related, manner to how it binds DNA; and (2) Est1 employs two separate pockets to bind distinct motifs of Cdc13. The N-terminal Cdc13-binding site of Est1 cooperates with the TLC1-Ku-Sir4 pathway for telomerase recruitment, whereas the C-terminal interface is dispensable for binding Est1 in vitro yet is nevertheless essential for telomere maintenance in vivo. Overall, our results integrate previous models and provide fundamentally valuable structural information regarding telomere biology.