In vivo visualization of type II plasmid segregation: bacterial actin. laments pushing plasmids

Type II par operons harness polymerization of the dynamically unstable actin- like protein ParM to segregate low- copy plasmids in rod- shaped bacteria. In this study, we use time- lapse. fluorescence microscopy to follow plasmid dynamics and ParM assembly in Escherichia coli. Plasmids lacking a par operon undergo con. fined diffusion with a diffusion constant of 5 x 10(-5) mu m(2)/ s and a confinement radius of 0.28 mu m. Single par-containing plasmids also move diffusively but with a larger diffusion constant ( 4 x 10- 4 mu m(2)/ s) and confinement radius ( 0.42 mu m). ParM. laments are dynamically unstable in vivo and form spindles that link pairs of par- containing plasmids and drive them rapidly ( 3.1 mu m/ min) toward opposite poles of the cell. After reaching the poles, ParM. laments rapidly and completely depolymerize. After ParM disassembly, segregated plasmids resume diffusive motion, often encountering each other many times and undergoing multiple rounds of ParM- dependent segregation in a single cell cycle. We propose that in addition to driving segregation, the par operon enables plasmids to search space and. find sister plasmids more effectively.