ParAB Partition Dynamics in Firmicutes: Nucleoid Bound ParA Captures and Tethers ParB-Plasmid Complexes

In Firmicutes, small homodimeric ParA-like (δ(2)) and ParB-like (ω(2)) proteins, in concert with cis-acting plasmid-borne parS and the host chromosome, secure stable plasmid inheritance in a growing bacterial population. This study shows that (ω:YFP)(2) binding to parS facilitates plasmid clustering in the cytosol. (δ:GFP)(2) requires ATP binding but not hydrolysis to localize onto the cell’s nucleoid as a fluorescent cloud. The interaction of (δ:CFP)(2) or δ(2) bound to the nucleoid with (ω:YFP)(2) foci facilitates plasmid capture, from a very broad distribution, towards the nucleoid and plasmid pairing. parS-bound ω(2) promotes redistribution of (δ:GFP)(2), leading to the dynamic release of (δ:GFP)(2) from the nucleoid, in a process favored by ATP hydrolysis and protein-protein interaction. (δD60A:GFP)(2), which binds but cannot hydrolyze ATP, also forms unstable complexes on the nucleoid. In the presence of ω(2), (δD60A:GFP)(2) accumulates foci or patched structures on the nucleoid. We propose that (δ:GFP)(2) binding to different nucleoid regions and to ω(2)-parS might generate (δ:GFP)(2) gradients that could direct plasmid movement. The iterative pairing and unpairing cycles may tether plasmids equidistantly on the nucleoid to ensure faithful plasmid segregation by a mechanism compatible with the diffusion-ratchet mechanism as proposed from in vitro reconstituted systems.