Abstract: E. coli bacterial strains encoding a restriction gene that degrades methylated DNA, and prevents cloning of genes expressing the methyltransferase responsible for methylation, are mutated by a chemical or physical mutagen, so as to make the restriction enzyme temperature sensitive. Mutant cells are rendered competent, and plasmids expected or known to contain genes encoding methyltransferase enzymes are introduced. The transformants grow at the permissive temperature, where the restriction enzyme system is inactivated due to the mutated gene. Successful clones, expressing a methyltransferase, can be quickly identified by those which grow at the permissive temperature, but not at the non-permissive temperature. The valuable methyltransferases, as well as restriction enzymes associated therewith, can accordingly be recovered in large quantity.