HORIZONTAL GENE-TRANSFER CONTRIBUTES TO THE WIDE DISTRIBUTION AND EVOLUTION OF TYPE-II RESTRICTION-MODIFICATION SYSTEMS

Restriction modification (RM) systems serve to protect bacteria agains t bacteriophages. They comprise a restriction endonuclease activity th at specifically cleaves DNA and a corresponding methyltransferase acti vity that specifically methylates the DNA, thereby protecting it from cleavage. Such systems are very common in bacteria. To fmd out whether the widespread distribution of RM systems is due to horizontal gene t ransfer, we have compared the codon usages of 29 type II RM systems wi th the average codon usage of their respective bacterial hosts. Pronou nced deviations in codon usage were found in six cases: EcoRI, EcoRV, KpnI, SinI, SmaI, and TthHB81. They are interpreted as evidence for ho rizontal gene transfer in these cases. As the methodology is expected to detect only one-fourth to one-third of all horizontal gene transfer events, this result implies that horizontal gene transfer had a consi derable influence on the distribution and evolution of RM systems. In all of these six cases the codon usage deviations of the restriction e nzyme genes are much more pronounced than those of the methyltransfera se genes. This result suggests that in these cases horizontal gene tra nsfer had occurred sequentially with the gene for the methyltransferas e being first acquired by the cell. This can be explained by the fact that an active restriction endonuclease is highly toxic in cells whose DNA is not protected from cleavage by a corresponding methyltransfera se.