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Molecular and Cellular Biology, February 2008, p. 897-906, Vol. 28, No. 3
0270-7306/08/$08.00+0     doi:10.1128/MCB.00524-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Rad51-Independent Interchromosomal Double-Strand Break Repair by Gene Conversion Requires Rad52 but Not Rad55, Rad57, or Dmc1 ^,

Department of Molecular Genetics and Microbiology and Cancer Research and Treatment Center, University of New Mexico School of Medicine, Albuquerque, New Mexico 87131

Received 26 March 2007/ Returned for modification 8 June 2007/ Accepted 5 November 2007

Homologous recombination (HR) is critical for DNA double-strand break (DSB) repair and genome stabilization. In yeast, HR is catalyzed by the Rad51 strand transferase and its "mediators," including the Rad52 single-strand DNA-annealing protein, two Rad51 paralogs (Rad55 and Rad57), and Rad54. A Rad51 homolog, Dmc1, is important for meiotic HR. In wild-type cells, most DSB repair results in gene conversion, a conservative HR outcome. Because Rad51 plays a central role in the homology search and strand invasion steps, DSBs either are not repaired or are repaired by nonconservative single-strand annealing or break-induced replication mechanisms in rad51 mutants. Although DSB repair by gene conversion in the absence of Rad51 has been reported for ectopic HR events (e.g., inverted repeats or between plasmids), Rad51 has been thought to be essential for DSB repair by conservative interchromosomal (allelic) gene conversion. Here, we demonstrate that DSBs stimulate gene conversion between homologous chromosomes (allelic conversion) by >30-fold in a rad51 mutant. We show that Rad51-independent allelic conversion and break-induced replication occur independently of Rad55, Rad57, and Dmc1 but require Rad52. Unlike DSB-induced events, spontaneous allelic conversion was detected in both rad51 and rad52 mutants, but not in a rad51 rad52 double mutant. The frequencies of crossovers associated with DSB-induced gene conversion were similar in the wild type and the rad51 mutant, but discontinuous conversion tracts were fivefold more frequent and tract lengths were more widely distributed in the rad51 mutant, indicating that heteroduplex DNA has an altered structure, or is processed differently, in the absence of Rad51.

Published ahead of print on 26 November 2007.

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