The role of the mismatch repair machinery in regulating mitotic and meiotic recombination between diverged sequences in yeast

Nonidentical recombination substrates recombine less efficiently than do identical substrates in yeast, and much of this inhibition can be attributed to action of the mismatch repair (MMR) machinery. In this study an intron-based inverted repeat assay system has been used to directly compare the rates of mitotic and meiotic recombination between pairs of 350-bp substrates varying from 82% to 100% in sequence identity. The recombination rate data indicate that sequence divergence impacts mitotic and meiotic recombination similarly, although subtle differences are evident. In addition to assessing recombination rates as a function of sequence divergence, the endpoints of mitotic and meiotic recombination events involving 94%-identical substrates were determined by DNA sequencing. The endpoint analysis indicates that the extent of meiotic heteroduplex DNA formed in a MMR-defective strain is 65% longer than that formed in a wild-type strain. These data are consistent with a model in which the MMR machinery interferes with the formation and/or extension of heteroduplex intermediates during recombination.