P1 plasmid partition: in vivo evidence for the ParA- and ParB-mediated formation of an anchored parS complex in the absence of a partner parS.

ParA and ParB proteins and cis-acting site, parS, are required to partition plasmid P1 faithfully to daughter cells. The process may initiate from plasmids paired by ParB at which recruited ParA then acts to effect the separation. We previously reported evidence for ParB-mediated pairing of parS sites on plasmids in the absence of ParA. In DNA gyrase-inhibited cells, the pairing prevented diffusion of transcription-generated positive supercoils. This supercoil trapping was almost entirely in plasmid dimers, where the location of the parS sites in cis facilitated their pairing. Here we show that the addition of ParA blocked supercoil diffusion also in plasmid monomers. The possibility that this result is attributed to an enhancement by ParA of ParB-mediated pairing in trans is consistent with our finding that ParA appeared to partially suppress the pairing defect of two mutant ParB proteins. However, enhanced pairing alone could not account for the diffusion barrier in plasmid monomers; it was manifest in monomers even when they were largely devoid of partners in the same cell. Apparently, ParA altered the ParB-parS complex such that it could no longer swivel, most likely by anchoring it, a reaction of probable relevance to partition.