Overlapping Functions of the <i>Saccharomyces cerevisiae</i> Mre11, Exo1 and Rad27 Nucleases in DNA Metabolism

MRE11 functions in several aspects of DNA metabolism, including meiotic recombination, double-strand break repair, and telomere maintenance. Although the purified protein exhibits 3' to 5' exonuclease and endonuclease activities in vitro, Mre11 is implicated in the 5' to 3' resection of duplex ends in vivo. The mre11-H125N mutation, which eliminates the nuclease activities of Mre11, causes an accumulation of unprocessed double-strand breaks (DSBs) in meiosis, but no defect in processing HO-induced DSBs in mitotic cells, suggesting the existence of redundant activities. Mutation of EXO1, which encodes a 5' to 3' exonuclease, was found to increase the ionizing radiation sensitivity of both mre11Delta and mre11-H125N strains, but the exo1 mre11-H125N strain showed normal kinetics of mating-type switching and was more radiation resistant than the mre11Delta strain. This suggests that other nucleases can compensate for loss of the Exo1 and Mre11 nucleases, but not of the Mre11-Rad50-Xrs2 complex. Deletion of RAD27, which encodes a flap endonuclease, causes inviability in mre11 strains. When mre11-H125N was combined with the leaky rad27-6, the double mutants were viable and no more gamma-ray sensitive than the mre11-H125N strain. This suggests that the double mutant defect is unlikely to be due to defective DSB processing.