HIGHLY MISMATCHED MOLECULES RESEMBLING RECOMBINATION INTERMEDIATES EFFICIENTLY TRANSFORM MISMATCH REPAIR PROFICIENT ESCHERICHIA-COLI

The ability of related DNAs to undergo recombination decreases with in creased sequence divergence. Mismatch repair has been proposed to be a key factor in preventing homeologous recombination; however, the cont ribution of mismatch repair is not universal. Although mismatch repair has been proposed to act by preventing strand exchange and/or inactiv ating multiply mismatched heteroduplexes, there has been no systematic study to determine at what step(s) in recombination mismatch repair a cts in vivo. Since heteroduplex is a commonly proposed intermediate in many models of recombination, we have investigated the consequences o f mismatch repair on plasmids that are multiply mismatched in heterodu plex structures that are similar to those that might arise during reco mbination. Plasmids containing multiply mismatched regions were transf ormed into wild-type and Mut(-) Escherichia coli mutants. There was on ly a 30-40% reduction in transformation of Mut(+) as compared to mutS and mutL strains for DNAs containing an 18% mismatched heteroduplex. T he products obtained from mutS hosts differed from those obtained from Mut(+) hosts in that there were many more colonies containing mixture s of two plasmids, due to survival of both strands of the heteroduplex . There were nearly 10 times more recombinants obtained from the mutS as compared to the wild-type host. Based on these results and those fr om other studies with E. coli and yeast, we propose that the preventio n of recombination between highly diverged DNAs may be al a step earli er than heteroduplex formation.