Plasmid recombination in a rad52 mutant of Saccharomyces cerevisiae.

Using plasmids capable of undergoing intramolecular recombination, we have compared the rates and the molecular outcomes of recombination events in a wild-type and a rad52 strain of Saccharomyces cerevisiae. The plasmids contain his3 heteroalleles oriented in either an inverted or a direct repeat. Inverted repeat plasmids recombine approximately 20-fold less frequently in the mutant than in the wild-type strain. Most events from both cell types have continuous coconversion tracts extending along one of the homologous segments. Reciprocal exchange occurs in fewer than 30% of events. Direct repeat plasmids recombine at rates comparable to those of inverted repeat plasmids in wild-type cells. Direct repeat conversion tracts are similar to inverted repeat conversion tracts in their continuity and length. Inverted and direct repeat plasmid recombination differ in two respects. First, rad52 does not affect the rate of direct repeat recombination as drastically as the rate of inverted repeat recombination. Second, direct repeat plasmids undergo crossing over more frequently than inverted repeat plasmids. In addition, crossovers constitute a larger fraction of mutant than wild-type direct repeat events. Many crossover events from both cell types are unusual in that the crossover HIS3 allele is within a plasmid containing the parental his3 heteroalleles.