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Originally published as Genetics Published Articles Ahead of Print on June 24, 2008.

Genetics, Vol. 179, 1807-1821, August 2008, Copyright © 2008
doi:10.1534/genetics.108.090654

Mutants Defective in Rad1-Rad10-Slx4 Exhibit a Unique Pattern of Viability During Mating-Type Switching in Saccharomyces cerevisiae

Amy M. Lyndaker, Tamara Goldfarb1 and Eric Alani2

Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14853-2703

2 Corresponding author: Department of Molecular Biology and Genetics, Cornell University, 459 Biotechnology Bldg., Ithaca, NY 14853-2703.
E-mail: eea3{at}cornell.edu

Efficient repair of DNA double-strand breaks (DSBs) requires the coordination of checkpoint signaling and enzymatic repair functions. To study these processes during gene conversion at a single chromosomal break, we monitored mating-type switching in Saccharomyces cerevisiae strains defective in the Rad1-Rad10-Slx4 complex. Rad1-Rad10 is a structure-specific endonuclease that removes 3' nonhomologous single-stranded ends that are generated during many recombination events. Slx4 is a known target of the DNA damage response that forms a complex with Rad1-Rad10 and is critical for 3'-end processing during repair of DSBs by single-strand annealing. We found that mutants lacking an intact Rad1-Rad10-Slx4 complex displayed RAD9- and MAD2-dependent cell cycle delays and decreased viability during mating-type switching. In particular, these mutants exhibited a unique pattern of dead and switched daughter cells arising from the same DSB-containing cell. Furthermore, we observed that mutations in post-replicative lesion bypass factors (mms2{Delta}, mph1{Delta}) resulted in decreased viability during mating-type switching and conferred shorter cell cycle delays in rad1{Delta} mutants. We conclude that Rad1-Rad10-Slx4 promotes efficient repair during gene conversion events involving a single 3' nonhomologous tail and propose that the rad1{Delta} and slx4{Delta} mutant phenotypes result from inefficient repair of a lesion at the MAT locus that is bypassed by replication-mediated repair.