Life cycle of restriction-modification gene complexes, powers in genome evolution

Increasing lines of evidence suggest that gene complexes encoding restriction modification (RM) enzymes may behave as selfish mobile genetic elements affecting genome stability and genome evolution. I here compare their hypothetical life cycles with those of temperate bacteriophage DNAs and other mobile elements. The restriction modification gene complexes may establish themselves in a new host avoiding cell killing. Threats on their maintenance would lead to cell killing. Upon some environmental signals, they would be induced to multiply. The multiplied RM gene complex may be released to the environment and taken up by other cells in naturally competent bacteria. In some aspects, they could be regarded as DNA viruses without a capsid, or DNA viroids. They may play the role of power giving order to the community of genes called the genome. (C) 2002 Elsevier Science B.V. All rights reserved.