We have characterized Escherichia coli DNA adenine methyltransferase, a critical regulator of bacterial virulence. Steady-state kinetics, product inhibition, and isotope exchange studies are consistent with a kinetic mechanism in which the cofactor S-adenosylmethionine binds first, followed by sequence-specific DNA binding and catalysis. The enzyme has a fast methyl transfer step followed by slower product release steps, and we directly demonstrate the competence of the enzyme cofactor complex. Methylation of adjacent GATC sites is distributive with DNA derived from a genetic element that controls the transcription of the adjacent genes. This indicates that the first methylation event is followed by enzyme release. The affinity of the enzyme for both DNA and S-adenosylmethionine was determined. Our studies provide a basis for further structural and functional analysis of this important enzyme and for the identification of inhibitors for potential therapeutic applications.