SINGLE-STRANDED-DNA ARISING AT TELOMERES IN CDC13 MUTANTS MAY CONSTITUTE A SPECIFIC SIGNAL FOR THE RAD9 CHECKPOINT

A cdc13 temperature-sensitive mutant of Saccharomyces cerevisiae arres ts in the G(2) phase of the cell cycle at the restrictive temperature as a result of DNA damage that activates the RAD9 checkpoint. The DNA lesions present after a failure of Cdc13p function appear to be locate d almost exclusively in telomere-proximal regions, on the basis of the profile of induced mitotic recombination. cdc13 rad9 cells dividing a t the restrictive temperature contain single-stranded DNA correspondin g to telomeric and telomere-proximal DNA sequences and eventually lose telomere-associated sequences. These results suggest that the CDC13 p roduct functions in telomere metabolism, either in the replication of telomeric DNA or in protecting telomeres from the double-strand break repair system. Moreover, since cdc13 rad9 cells divide at a wild-type rate for several divisions at the restrictive temperature while cdc13 RAD9 cells arrest in G(2), these results also suggest that single-stra nded DNA may be a specific signal for the RAD9 checkpoint.