A previous study showed that in mammals the pathways leading to synthesis and post-synthetic modification of DNA employ methionine as common donor of atoms: the carbon coming from the methyl group of this amino acid is needed for replication; its entire methyl group is needed to build m(5)C on semiconservatively newly replicating chains. This work showed that the two pathways originate in bacteria where an enzymatic system forms, on DNA, m(6)A in addition to m(5)C. The formation rate of m(6)A gradually decreased during the bacterial CGC, while that of m(5)C reached an optimum in its middle. This shift suggested that the dcm and dam methyltransferase activities, as well as the activities of the methyltransferase moieties of the RM enzymes, are uncoupled.