CAA reactivity of cytosines in cognate (GCGC/GMGC) and mismatch (GCGC/GMCC) 25-mer DNA duplexes upon interaction with M.HhaI (or its Q237G mutant) and cofactor product SAH. Autoradiograph of degradation products of the 25-mer duplexes 5′-labeled on the target (upper) strand after separation by electrophoresis on a 15% sequencing gel. Lane 1, G + A nucleotide cleavage marker; lanes 2–7, reactions with 200 mM CAA for 45 min at 20°C, containing 300 nM protein, 280 nM DNA and 0.1 mM SAH as follows: lane 2, upper GCGC strand alone (treated at 37°C); lane 3, GCGC/GMGC; lane 4, GCGC/GMGC + M.HhaI, lane 5, GCGC/GMGC + M.HhaI + SAH; lane 6, GCGC/GMGC + M.HhaI(Q237G); lane 7, GCGC/GMGC +M.HhaI (Q237G) + SAH; lanes 8–11, CAA reactions containing 30 nM protein and 10 nM DNA as follows: lane 8, GCGC/GMGC; lane 9, GCGC/GMGC + M.HhaI; lane 10, GCGC/GMCC; lane 11, GCGC/GMCC + M.HhaI.