The partition system of multidrug resistance plasmid TP228 includes a novel protein that epitomizes an evolutionarily distinct subgroup of the ParA superfamily

The segregational stability of bacterial, low-copy-number plasmids is promo ted primarily by active partition, The plasmid-specified components of the prototypical pi plasmid partition system consist of two proteins, ParA (44. 3 kDa) and pars (38.5 kDa), which, in conjunction with integration host fac tor, form a nucleoprotein complex at the plasmid partition site, parS. This complex is the probable substrate for the directed temporal and spatial in tracellular movement of plasmids before cell division. The genetic organiza tion of the partition cassette of the multidrug resistance plasmid TP228 di ffers markedly from that of the pi paradigm. The TP228 system includes a no vel member (ParF; 22.0 kDa) of the ParA superfamily of ATPases, of which th e pi ParA protein is the archetype. However, the ParF protein and its immed iate relatives form a discrete subgroup of the ParA superfamily, which evol utionarily is more related to the MinD subgroup of cell division proteins t han to ParA of P1. The TP228 and P1 partition modules differ further in tha t the former does not include a parS homologue, but does specify a protein (ParG; 8.6 kDa) unrelated to pare. Homologues of the parF gene are widely d isseminated on eubacterial genomes, suggesting that ParF-mediated partition may be a common mechanism by which plasmid segregational stability is achi eved.