Methylation assay. (a) Protection from R.AhdI endonuclease indicates methylation of the plasmid by M.AhdI. Lane 1, 1 kb marker; lane 2, uncut plasmid; lane 3, linearised plasmid; lane 4, linearised plasmid cut with R.AhdI; lane 5, linearised plasmid challenged with R.AhdI after incubation with heat killed M.AhdI; lane 6, linearised plasmid challenged with R.AhdI after incubation with native M.AhdI; lanes 7–30, time course of methylation with samples taken every 5 min. (b) Enzyme progression curve, showing the concentration of unmethylated DNA (nM) against incubation time with the M.AhdI enzyme. The data have been fitted to the integrated form of the Michaelis–Menten equation.