DOUBLE-STRAND BREAK REPAIR IN THE ABSENCE OF RAD51 IN YEAST - A POSSIBLE ROLE FOR BREAK-INDUCED DNA-REPLICATION

In wild-type diploid cells of Saccharomyces cerevisiae, an HO endonucl ease induced double-strand break (DSB) at the MAT locus can be efficie ntly repaired by gene conversion using the homologous chromosome seque nces. Repair of the broken chromosome was nearly eliminated in rad52 D elta diploids; 99% lost the broken chromosome. However, in rad51 Delta diploids, the broken chromosomes were repaired approximately 35% of t he time. None of these repair events were simple gene conversions or g ene conversions with an associated crossover; instead, they created di ploids homozygous for the MAT locus and all markers in the 100-kb regi on distal to the site of the DSB. In rad51 Delta diploids, the broken chromosome can apparently be inherited for several generations, as man y of these repair events are found as sectored colonies, with one part being repaired and the other part having lost the broken chromosome. Similar events occur in about 2% of wild-type cells. We propose that a broken chromosome end can invade a homologous template in the absence of RAD51 and initiate DNA replication that may extend to the telomere , 100 or more kb away. Such break-induced replication appears to be si milar to recombination-initiated replication in bacteria.