ALTERED PARA PARTITION PROTEINS OF PLASMID P1 ACT VIA THE PARTITION SITE TO BLOCK PLASMID PROPAGATION

The partition system of the P1 plasmid, P1par, consists of the ParA an d ParB proteins and a cis-acting site, parS. It is responsible for the orderly segregation of plasmid copies to daughter cells. Plasmids wit h null mutations in parA or parB replicate normally, but missegregate. ParB binds specifically to the parS site, but the role of ParA and it s ATPase activity in partition is unclear. We describe a novel class o f parA mutants that cannot be established or maintained as plasmids un less complemented by the wild-type gene. One, parAM314I, is conditiona l: it can be maintained in cells in minimal medium but cannot be estab lished in cells growing in L broth. The lack of plasmid propagation in L broth-grown cells was shown to be caused by a ParB-dependent activi ty of the mutant ParA protein that blocks plasmid propagation by an in teraction at the parS site. Thus, ParA acts to modify the ParB-parS co mplex, probably by binding to it. Partition is thought to involve sele ction of pairs of plasmids before segregation, either by physical pair ing of copies or by binding of copies to paired host sites. We suggest that ParA is involved in this reaction and that the mutant ParA prote in forms paired complexes that cannot unpair.