A dual role for substrate S-adenosyl-L-methionine in the methylation reaction with bacteriophage T4 Dam DNA-[ N6-adenine]-methyltransferase

The fluorescence of P-aminopurine ((2)A)-substituted duplexes (contained in the GATC target site) was investigated by titration with T4 Dam DNA-(N6-ad enine)-methyltransferase. With an unmethylated target ((2)A/A duplex) or it s methylated derivative ((2)A/(m)A duplex), T4 Dam produced up to a 50-fold increase in fluorescence, consistent with (2)A being flipped out of the DN A helix. Though neither S-adenosyl-L-homocysteine nor sinefungin had any si gnificant effect, addition of substrate S-adenosyl-L-methionine (AdoMet) sh arply reduced the Dam-induced fluorescence with these complexes. In contras t, AdoMet had no effect on the fluorescence increase produced with an (2)A/ (2)A double-substituted duplex. Since the (2)A/(m)A duplex cannot be methyl ated, the AdoMet-induced decrease in fluorescence cannot be due to methylat ion per se, We propose that T4 Dam alone randomly binds to the asymmetric ( 2)A/A and (2)A/(m)A duplexes, and that AdoMet induces an allosteric T4 Dam conformational change that promotes reorientation of the enzyme to the stra nd containing the native base. Thus, AdoMet increases enzyme binding-specif icity, in addition to serving as the methyl donor, The results of pre-stead y-state methylation kinetics are consistent with this model.