Mutation of msh-2 in Neurospora crassa does not reduce the incidence of recombinants with multiple patches of donor chromosome sequence.

The Neurospora homologue msh-2 of the Escherichia coli mismatch repair gene mutS was mutated by repeat-induced point mutation (RIP) of a 1.9-kb duplication covering 1661bp of the coding sequence and 302 bp 5' of the gene. msh-2(RIP-LK1) exhibited a mutator phenotype conferring a 17-fold increase in the frequency of spontaneous mitotic reversion of his-3 allele K458. In msh-2(RIP-LK1) homozygotes, recombination frequency at the his-3 locus increased up to 2.9-fold over that in msh-2(+) diploids. Progeny of crosses homozygous msh-2(RIP-LK1), like those from crosses homozygous msh-2(+) frequently had multiple patches of donor chromosome sequence, suggesting that patchiness in msh-2(+) crosses is not explained by incomplete repair of heteroduplex DNA by MSH-2. These findings are consistent with data from the analysis of events in a Neurospora translocation heterozygote that suggested multiple patches of donor chromosome sequence arising during recombination reflect multiple template switches during DNA repair synthesis.