Mismatch Repair Inhibits Homeologous Recombination via Coordinated Directional Unwinding of Trapped DNA Structures

• MutS and MutL trap strand-exchange intermediates during homeologous recombination
• Trapping involves binding to mismatches in heteroduplex DNA and to displaced ssDNA
• Trapping prevents strand exchange by imposing rotational constraints
• UvrD is recruited to directionally resolve the trapped recombination intermediates

SummaryHomeologous recombination between divergent DNA sequences is inhibited by DNA mismatch repair. In Escherichia coli, MutS and MutL respond to DNA mismatches within recombination intermediates and prevent strand exchange via an unknown mechanism. Here, using purified proteins and DNA substrates, we find that in addition to mismatches within the heteroduplex region, secondary structures within the displaced single-stranded DNA formed during branch migration within the recombination intermediate are involved in the inhibition. We present a model that explains how higher-order complex formation of MutS, MutL, and DNA blocks branch migration by preventing rotation of the DNA strands within the recombination intermediate. Furthermore, we find that the helicase UvrD is recruited to directionally resolve these trapped intermediates toward DNA substrates. Thus, our results explain on a mechanistic level how the coordinated action between MutS, MutL, and UvrD prevents homeologous recombination and maintains genome stability.