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Molecular and Cellular Biology, June 2003, p. 3721-3734, Vol. 23, No. 11
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.11.3721-3734.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

The Rad51 Pathway of Telomerase-Independent Maintenance of Telomeres Can Amplify TG_1-3 Sequences in yku and cdc13 Mutants of Saccharomyces cerevisiae

Ecole Normale Supérieure de Lyon, UMR CNRS 5665, 69364 Lyon, France

Received 16 September 2002/ Returned for modification 25 October 2002/ Accepted 13 March 2003

In the yeast Saccharomyces cerevisiae, Cdc13, Yku, and telomerase define three parallel pathways for telomere end protection that prevent chromosome instability and death by senescence. We report here that cdc13-1 yku70 mutants generated telomere deprotection-resistant cells that, in contrast with telomerase-negative senescent cells, did not display classical crisis events. cdc13-1 yku70 cells survived telomere deprotection by exclusively amplifying TG_1-3 repeats (type II recombination). In a background lacking telomerase (tlc1), this process predominated over type I recombination (amplification of subtelomeric Y' sequences). Strikingly, inactivation of the Rad50/Rad59 pathway (which is normally required for type II recombination) in cdc13-1 yku70 or yku70 tlc1 mutants, but also in cdc13-1 YKU70⁺ tlc1 mutants, still permitted type II recombination, but this process was now entirely dependent on the Rad51 pathway. In addition, delayed senescence was observed in cdc13-1 yku70 rad51 and cdc13-1 tlc1 rad51 cells. These results demonstrate that in wild-type cells, masking by Cdc13 and Yku prevents the Rad51 pathway from amplifying telomeric TG_1-3 sequences. They also suggest that Rad51 is more efficient than Rad50 in amplifying the sequences left uncovered by the absence of Cdc13 or Yku70.

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