ParAB partition dynamics in firmicutes: Nucleoid bound ParA captures and tethers ParB-plasmid complexes

Abstract

In Firmicutes, small homodimeric ParA-like (δ 2) and ParB-like (υ 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 (υ:YFP) 2 binding to parSfacilitates 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 (δ:YFP) 2 foci facilitates plasmid capture, from a very broad distribution, towards the nucleoid and plasmid pairing. parS-bound υ 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 υ 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 υ 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.