MutS and MutL activate DNA helicase II in a mismatch-dependent manner.

MutS, MutL, and DNA helicase II are required for the mismatch-provoked excision step that occurs during Escherichia coli methyl-directed mismatch repair. In this study MutL is shown to enhance the unwinding activity of DNA helicase II more than 10-fold on a conventional helicase substrate in which a 35-residue oligonucleotide is annealed to a M13 circular single-stranded phage DNA under conditions where the two proteins are present at approximately molar stoichiometry with respect to the substrate. MutS- and MutL-dependent activation of DNA helicase II has also been demonstrated with a model substrate in which a 138-residue oligonucleotide was hybridized to a 138-nucleotide gap in an otherwise duplex 7,100-base pair circular DNA. Displacement of the oligonucleotide requires MutS, MutL, DNA helicase II, and ATP and is dependent on the presence of a mismatch within the hybrid region. Although DNA helicase II and Rep helicase share substantial sequence homology and features of mechanism, Rep helicase is inactive in this reaction.