Site-Specific Recombination

Many laboratory strains of E. coli have been mutated to be deficient in homologous recombination. This reduces the frequency with which genes cloned in multicopy plas-mids undergo rearrangements in these strains. In contrast, most wild-type bacteria are recombination proficient and this is critical for bacterial DNA repair and evolution (53). As plasmid copies are identical, homologous recombination in wild-type bacteria can convert plasmid monomers to dimers or higher-order species. The complete dimer-ization of a plasmid population within a cell will halve the number of plasmids avail­able for partition at cell division and thereby contribute to plasmid segregational instability. Furthermore, because dimers have two replication origins, they may be more favored for replication than plasmids with a single origin, which may further skew intracellular plasmid distribution toward dimeric forms. The formation of trimers and other multimers will have an even more profound effect on plasmid segregation (54).

Fig. 4. The contribution of accessory stability mechanisms to plasmid maintenance.

Active partitioning