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

<div><p>In Firmicutes, small homodimeric ParA-like (δ₂) and ParB-like (ω₂) proteins, in concert with cis-acting plasmid-borne <i>parS</i> and the host chromosome, secure stable plasmid inheritance in a growing bacterial population. This study shows that (ω:YFP)₂ binding to <i>parS </i>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 δ₂ bound to the nucleoid with (ω:YFP)₂ foci facilitates plasmid capture, from a very broad distribution, towards the nucleoid and plasmid pairing. <i>parS</i>-bound ω₂ 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 ω₂, (δD60A:GFP)₂ accumulates foci or patched structures on the nucleoid. We propose that (δ:GFP)₂ binding to different nucleoid regions and to ω₂-<i>parS</i> 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 <i>in vitro</i> reconstituted systems.</p>