8-AMino[1”-(N”-dansyl)-4”-aMinobutyl]-5'-(1-aziridinyl)-5'-deoxy Adenosine can be used as a fluorescent cofactor reagent for the sequence-specific labeling of DNA methyltransferase from Thermus aquaticus (M*Taql). I n addition, the reagent can be used to sequence-specifically label plasmid DNA in a M*Taql-catalyzed reaction.