ROLE OF RECIPROCAL EXCHANGE, ONE-ENDED INVASION CROSSOVER AND SINGLE-STRAND ANNEALING ON INVERTED AND DIRECT REPEAT RECOMBINATION IN YEAST - DIFFERENT REQUIREMENTS FOR THE RAD1, RAD10, AND RAD52 GENES

We have constructed novel DNA substrates (one inverted and three direc t repeats) based on the same 0.6-kb repeat sequence to study deletions and inversions in Saccharomyces cervisiae. Spontaneous deletions occu r six to eight times more frequently than inversions, irrespective of the distance between the repeats. This difference can be explained by the observation that deletion events can be mediated by a recombinatio n mechanism that can initiate within the intervening sequence of the r epeats. Spontaneous and double-strand break (DSB)-induced deletions oc cur as RAD52-dependent and RAD52-dependent events. Those deletion even ts initiated through a DSB in the unique intervening sequence require the Rad1/Rad10 endonuclease only if the break is distantly located fro m the flanking DNA repeats. We propose that deletions can occur as thr ee types of recombination events: the conservative RAD52- dependent re ciprocal exchange and the nonconservative events, one-ended invasion c rossover, and single-strand annealing (SSA). We suggest that one-ended invasion is RAD52 dependent, whereas SSA is RAD52 independent. Wherea s deletions, like inversions, occur through reciprocal exchange, delet ions can also occur through SSA or one-ended invasion. We propose that the contribution of reciprocal exchange and one-ended invasion crosso ver vs. SSA events to overall spontaneous deletions is a feature speci fic for each repeat system, determined by the initiation event and the availability of the Rad52 protein. We discuss the role of the Rad1/Ra d10 endonuclease on the initial steps of one-ended invasion crossover and SSA as a function of the location of the initiation event relative to the repeats. We also show that the frequency of recombination betw een repeats is the same independent of their location (whether on circ ular plasmids, linear minichromosomes, or natural chromosomes) and har e similar RAD52 dependence.