Expansions and contractions in 36-bp minisatellites by gene conversion in yeast

The instability of simple tandem repeats, such as human minisatellite loci, has been suggested to arise by gene conversions. In Saccharomyces cerevisi ae, a double-strand break (DSB) was created by the HO endonuclease so that DNA polymerases associated with gap repair must traverse an artificial mini satellite of perfect 36-bp repeats or a yeast Y ' minisatellite containing diverged 36-bp repeats. Gene conversions are frequently accompanied by chan ges in repeat number when tile template contains perfect repeats. When the ends of the DSB have nonhomologous tails of 47 and 70 nucleotides that must be removed before repair DNA synthesis can begin, 16% of gene conversions had rearrangements, most of which were contractions, almost always in the r ecipient locus. When efficient removal of nonhomologous tails was prevented in rad1 and msh2 strains, repair was reduced 10-fold, but among survivors there was a 10-fold reduction in contractions. Half the remaining events we re expansions. A similar decrease in the contraction rate was observed when the template was modified so that DSB ends were homologous to the template ; and here, too, half of the remaining rearrangements were expansions. In t his case, efficient repair does not require RAD1 and MSH2, consistent with our previous observations. In addition, without nonhomologous DSB ends, msh 2 and rad1 mutations did not affect the frequency or the distribution of re arrangements. We conclude that the presence of nonhomologous ends alters ti le mechanism of DSB repair, likely through early recruitment of repair prot eins including Msh2p and Rad1p, resulting in more frequent contractions of repeated sequences.