Evolution of R plasmids by replicon fusion.

The occurrence of plasmid-plasmid fusions in nature and their possible influence on the spread of bacterial drug resistance is considered. Three ways in which replicon fusions can be formed are described; specifically fusions mediated by IS elements, "one-ended transposition" systems derived from Tn3 and like transposons, and pUB2380, a prototype for a new type of transposition. Non-transposition based systems which mediate replicon fusion are also briefly considered.