Asymmetric Recognition of DNA Local Distortion

Crystal structures of bacterial MutS homodimers bound to mismatched DNA reveal asymmetric interactions of the two subunits with DNA. A phenylalanine and glutamate of one subunit make mismatched base-specific interactions, and residues of both subunits contact the DNA backbone surrounding the mismatched base, but asymmetrically. A number of amino acids in MutS that contact the DNA are conserved in the eukaryotic Msh2-Msh6 heterodimer. We report here that yeast strains with amino acids substituted for residues inferred to interact with the DNA backbone or mismatched base have elevated spontaneous mutation rates consistent with defective mismatch repair. Purified Msh2-Msh6 with substitutions in the conserved Phe and Glu in Msh6 thought to stack or hydrogen bond, respectively, with the mismatched base do have reduced DNA binding affinity but normal ATPase activity. Moreover, wild-type Msh2-Msh6 binds with lower affinity to mismatches with thymine replaced by difluorotoluene, which lacks the ability to hydrogen bond. The results suggest that yeast Msh2-Msh6 interacts asymmetrically with the DNA through base-specific stacking and hydrogen bonding interactions and backbone contacts. The importance of these contacts decreases with increasing distance from the mismatch, implying that interactions at and near the mismatch are important for binding in a kinked DNA conformation.