Kinetics of plasmid segregation in Escherichia coli

Low copy-number bacterial replicons occupy specific locations in their host cells. Production of a GFP-Lac repressor hybrid protein in cells carrying F or P1 plasmids tagged with a lac operator array reveals that in smaller (younger) cells these plasmids are seen mainly as a single fluorescent focus at mid-cell, whereas larger cells tend to have two foci, one at each quarter-cell position. Duplication of the central focus is presumed to represent active partition of plasmid copies. We report here our investigation by time-lapse microscopy of the subsequent movement of these copies to the quarter positions. Following duplication of the central focus, the new foci migrated rapidly and directly to their quarter-cell destinations, where they remained until the next cell cycle. The speed of movement was about five times faster than poleward migration of oriC and 50 times faster than cell elongation. Aberrant positioning of mini-F lacking its sopC centromere demonstrated the requirement for the partition system in this localization process. From the measured number of F plasmid copies per cell it appears that each migrating focus contains two or more plasmid molecules. The molecular basis of this clustering, and evidence for phasing of the partition event in the cell cycle, are discussed.