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 id entical substrates in yeast, and much of this inhibition can be attributed to action of the mismatch repair (MMR) machinery. In this study an intron-b ased inverted repeat assay system has been used to directly compare the rat es of mitotic and meiotic recombination between pairs of 356-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 assessi ng recombination rates as a function of sequence divergence, the endpoints of mitotic and meiotic recombination events involving 94%-identical substra tes 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 consiste nt with a model in which the MMR machinery interferes with the formation an d/or extension of heteroduplex intermediates during recombination.