Individually inactive N- and C-terminal fragments of the m5C-methyltransferase M.BspRI can complement each other resulting in specific, in vivo methylation of the DNA. This was shown by cloning the coding regions for N- and C-terminal parts of the enzyme in compatible plasmids and co-transforming them into E.coli cells. The enzyme could be detached at several different sites, producing either non-overlapping or partially overlapping fragments capable of complementation. Reconstitution of the active methyltransferase from inactive fragments was demonstrated in vitro, as well. Another GGCC-specific methyltransferase, M.BsuRI, showed a similar complementation phenomenon. Moreover, interspecies complementation was observed between appropriate fragments of the two closely related enzymes M.BspRI and M.BsuRI. Fragments of structurally and functionally more different methyltransferases were unable to complement each other.