Evolution of type II DNA methyltransferases. A gene duplication model.

On the basis of consensus sequences, which had previously been defined for two groups of closely related cytosine-specific and adenine-specific DNA methyltransferases, homologies can be detected that indicate a common origin for these proteins. Intramolecular comparisons of several of these enzymes reveal homology relationships, which suggests that gene duplication is a phylogenetic principle in the evolution of the Mtases. One or two duplications of an ancestral gene encoding a 12,000 to 16,000 Mr protein, followed by divergent evolution, may have led to very different protein structures and could explain the differences in amino acid sequences, molecular weights and biochemical properties. Intermolecular and intramolecular homologies were also recognized in type II restriction endonucleases, suggesting a very similar evolutionary pathway.