MUTATIONS IN XRS2 AND RAD50 DELAY BUT DO NOT PREVENT MATING-TYPE SWITCHING IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, a large number of genes in the RAD52 epis tasis group has been implicated in the repair of chromosomal double-st rand breaks and in both mitotic and meiotic homologous recombination. While most of these genes are essential for yeast mating-type (MAT) ge ne switching, neither RAD50 nor XRS2 is required to complete this spec ialized mitotic gene conversion process. Using a galactose-inducible H O endonuclease gene to initiate MAT switching, we have examined the ef fect of null mutations of RAD50 and of XRS2 on intermediate steps of t his recombination event. Both rad50 and xrs2 mutants exhibit a marked delay in the completion of snitching. Both mutations reduce the extent of 5'-to-3' degradation from the end of the HO-created double-strand break. The steps of initial strand invasion and new DNA synthesis are delayed by approximately 30 min in mutant cells. However, later events are still further delayed, suggesting that XRS2 and RAD50 affect more than one step in the process. In the rad50 xrs2 double mutant, the co mpletion of MAT switching is delayed more than in either Single mutant , without reducing the overall efficiency of the process. The XRS2 gen e encodes an 854-amino-acid protein with no obvious similarity to the Rad50 protein or to any other protein in the database. Overexpression of RAD50 does not complement the defects in xrs2 or vice versa.