Plasmid Evolution: Plasmids Are Modular Elements

Whole genome and plasmid-specific sequencing projects have recently begun to provide fascinating glimpses into the genetic organization and evolution of plasmids. These studies have revealed that plasmids, particularly large plasmids, are commonly constructed in a modular fashion by the recombination activities of transposons, inser­tion sequences, bacteriophages, and smaller plasmids (72). For example, the backbone

Fig. 5. Simplified representation of the relative distribution of transposable elements (gray boxes) and putative virulence genes (filled arcs) on the pO157 virulence plasmid of E. coli. Replicons, one of which is apparently interrupted, are shown as white boxes. For clarity, the locations of partition, conjugation and other genes are not shown. (Adapted from ref. 73.)

of the 92-kbp virulence plasmid of E. coli O157 bears a striking resemblance to that of the F plasmid. However, this backbone is interrupted by a number of regions contain­ing putative virulence genes (see Fig. 5) (73). These virulence patches are framed by intact insertion sequences or insertion-sequence remnants,