MutS F36A mutant binds G4 DNA but not G-T mismatches. (A) Mobility shift demonstrating that F36 is required for mismatch binding. From left to right, lanes 1 and 6 contain no MutS protein. Lanes 2–5 and 7–10 contain increasing amounts (19–150 nM) of WT-MutS and F36A-MutS, respectively, obtained by 1:1 serial dilution. Each lane contains 20 fmols radio labeled G-T mismatch substrate. B, oligonucleotide bound by MutS or F36A; U, unbound. (B) F36 is dispensable for MutS binding G4. Far left lane “0”, negative control containing no MutS protein; lane 2 “MutS”, positive control containing 75 nM WT-MutS, lanes 3–8 contain increasing amounts of F36A-MutS from 4.7 to 150 nM, obtained by 1:1 serial dilution. Each lane contains 20 fmols of radiolabeled G4. (C) Graph comparing WT-MutS and F36A-MutS binding to TP-G4 DNA. White columns, F36A-MutS; grey columns, WT-MutS. Data (Additional file 3) represent the mean of three independent experiments with standard error.