Inhibition of the alternative lengthening of telomeres pathway by subtelomeric sequences in Saccharomyces cerevisiae

In the budding yeast Saccharomyces cerevisiae, telomerase is constitutively active and is essential for chromosome end protection and illimited proliferation of cell populations. However, upon inactivation of telomerase, alternative mechanims of telomere maintenance allow proliferation of only extremely rare survivors. S. cerevisiae type I and type II survivors differ by the nature of the donor sequences used for repair by homologous recombination of the uncapped terminal TG(1-3) telomeric sequences. Type I amplifies the subtelomeric Y' sequences and is more efficient than type II, which amplifies the terminal TG(1-3) repeats. However, type II survivors grow faster than type I survivors and can easily outgrow them in liquid cultures. The mechanistic interest of studying S. cerevisiae telomeric recombination is reinforced by the fact that type II recombination is the equivalent of the alternative lengthening of telomeres (ALT) pathway that is used by 5-15 % of cancer types as an alternative to telomerase reactivation. In budding yeast, only around half of the 32 telomeres harbor Y' subtelomeric elements. We report here that in strains harboring Y' elements on all telomeres, type II survivors are not observed, most likely due to an increase in the efficiency of type I recombination. However, in a temperature-sensitive cdc13-1 mutant grown at semi-permissive temperature, the increased amount of telomeric TG(1-3) repeats could overcome type II inhibition by the subtelomeric Y' sequences. Strikingly, in the 100 % Y' strain the replicative senescence crisis normally provoked by inactivation of telomerase completely disappeared and the severity of the crisis was proportional to the percentage of chromosome-ends lacking Y' subtelomeric sequences. The present study highlights the fact that the nature of subtelomeric elements can influence the selection of the pathway of telomere maintenance by recombination, as well as the response of the cell to telomeric damage caused by telomerase inactivation.