ParAB-mediated intermolecular association of plasmid P1 parS Sites

Abstract The P1 plasmid partition system depends on ParA–ParB proteins acting on centromere-like parS sites for a faithful plasmid segregation during the Escherichia coli cell cycle. In vivo we placed parS into host E. coli chromosome and on a Sop+ F plasmid and found that the stability of a P1 plasmid deleted for parA–parB could be partially restored when parB was expressed in trans. In vitro, parS , conjugated to magnetic beads could capture free parS DNA fragment in presence of ParB. In vitro, ParA stimulated ParB-mediated association of intermolecular parS sites in an ATP-dependent manner. However, in the presence of ADP, ParA reduced ParB-mediated pairing to levels below that seen by ParB alone. ParB of P1 pairs the parS sites of plasmids in vivo and fragments in vitro. Our findings support a model whereby ParB complexes P1 plasmids, ParA–ATP stimulates this interaction and ParA–ADP inhibits ParB pairing activity in a parS -independent manner.