Molecular Anatomy of ParA-ParA and ParA-ParB Interactions during Plasmid Partitioning.

Firmicutes multidrug resistance inc18 plasmids encode parS sites and two small homodimeric ParA-like (δ2) and ParB-like (ω2) proteins to ensure faithful segregation. Protein ω2 binds to parS DNA, forming a short left-handed helix wrapped around the full parS, and interacts with δ2. Protein δ2 interacts with ω2 and, in the ATP-bound form, binds to nonspecific DNA (nsDNA), forming small clusters. Here, we have mapped the ω2·δ2 and δ2·δ2 interacting domains in the δ2 that are adjacent to but distinct from each other. The δ2 nsDNA binding domain is essential for stimulation of ω2·parS-mediated ATP hydrolysis. From the data presented here, we propose that δ2 interacts with ATP, nsDNA, and with ω2 bound to parS at near equimolar concentrations, facilitating a δ2 structural transition. This δ2 "activated" state overcomes its impediment in ATP hydrolysis, with the subsequent release of both of the proteins from nsDNA (plasmid unpairing).