Xrs2 is a member of the MRX complex (Mre11/Rad50/Xrs2) in Saccharomyces cerevisiae. In this study we demonstrate the important role of the MRX complex and in more detail of Xrs2 for the repair of radiation-induced chromosomal double-strand breaks by pulsed field gel electrophoresis. By using a newly designed in vivo plasmid-chromosome recombination system, we could show that gap repair efficiency and the association with crossovers were reduced in the MRX null mutants, but repair accuracy was unaffected. For these processes, an intact Mre11-binding domain of Xrs2 is crucial, whereas the FHA- and BRCT-domains as well as the Tel1-binding domain of Xrs2 are dispensable. Obviously, the Mre11-binding domain of the Xrs2 protein is crucial for the analysed functions and our results suggest a new role of the MRX complex for the formation of crossovers. Analysis of double mutants showed that the phenotype of the Deltaxrs2 null mutant concerning the crossover frequency is dominant over the phenotypes of Deltasrs2 and Deltasgs1 null mutants. Thus, the complex seems to be involved in early steps of double-strand break and gap repair, and we propose that it has a regulatory role for the selection of homologous recombination pathways.
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