Plasmid Replication

Plasmids, like chromosomes, are replicated during the bacterial cell cycle so that the new cells can each be provided with at least one plasmid copy at cell division (41). To this end, plasmids have developed a number of strategies to initiate DNA replica­tion but have mostly co-opted the host polymerization machinery (42) for subsequent stages of DNA synthesis, thereby minimizing the amount of plasmid-encoded infor­mation required for their replication. Small plasmids have been identified which con­sist of a replicon and very little extraneous DNA sequences (42a). These, and other cryptic plasmids, can be viewed as purely selfish genetic elements as they apparently provide no advantage to their host. However, they may exclude related, invading plas-mids from the host or may function as the core of lager plasmids which will evolve in the future. Large plasmids often contain multiple replicons dispersed at different loca­tions on the plasmid or express different forms of a replication protein. These phe­nomena may reflect the different replication requirements of a plasmid that can exist in more than one bacterial host (43).