EXPANSIONS AND CONTRACTIONS IN A TANDEM REPEAT INDUCED BY DOUBLE-STRAND BREAK REPAIR

Repair of a double-strand break (DSB) in yeast can induce very frequen t expansions and contractions in a tandem array of 375-bp repeats. The se results strongly suggest that DSB repair can be a major source of a mplification of tandemly repeated sequences. Most of the DSB repair ev ents are not associated with crossover. Rearrangements appear in 50% o f these repaired recipient molecules. In contrast, the donor template nearly always remains unchanged. Among the rare crossover events, simi lar rearrangements are found. These results cannot readily be explaine d by the gap repair model of Szostak et al. (J. W. Szostak, T. L. Orr- Weaver, R. J. Rothstein, and F. W. Stahl, Cell 33:25-35, 1983) but can be explained by synthesis-dependent strand annealing (SDSA) models th at allow for crossover. Support for SDSA models is provided by a demon stration that a single DSB repair event can use two donor templates lo cated on two different chromosomes.