Active Partition Systems

Following plasmid replication, active partitioning systems position the plasmids appropriately within the cell such that at cell division, each of the new cells acquires at least one copy of the plasmid (see Fig. 4). The most well studied active partition sys­tem is, arguably, that of the P1 plasmid in E. coli (50,51). The plasmid located compo­nents of this system are organized in a cassette that consists of an autoregulated operon containing the parA and parB genes and a downstream cis-acting sequence, parS. The ParA and ParB proteins and a host protein, integration host factor, form a nucleopro-tein complex at parS that is presumed to interact with an unknown host partitioning apparatus. This complex guides the tethered P1 plasmid copies to the one-quarter and three-quarter cell-length positions following replication at the midcell. The plasmids remain at these positions as the bacterial cell elongates. When the cell divides at its center the plasmids are again at the midpoint positions of the new cells and the cycles of replication and partition are repeated (see Fig. 4).

Active partition systems are widely distributed among low-copy-number bacterial plasmids and homologous systems are likely to be implicated in chromosome partition in many bacteria (52).