FIGURE 1 MutS and MutL are required to activate MutH, which cleaves the unmethylated strand at a GATC site. (a) The DNA substrate contained a mismatch and a GATC sequence methylated on either the viral (V) strand or the complementary (C) strand. The MboI restriction site (in GATC) is shown. (b) Gel analysis of the reaction products. Proteins added to the reactions in each experiment are indicated at the top of the gel (H, L, and S represent MutH, MutL, and MutS), with MboI fragments as control. DNAs were detected by transfer to a membrane and use of a [32P]DNA oligonucleotide complementary to either the V or the C strand, to produce the autoradiograph.