Effect of growth rate and incC mutation on symmetric plasmid distribution by the IncP-1 partitioning apparatus

The incC and korB genes of IncP-1 plasmid RK2 encode homologues of ubiquito us ParA and pars partitioning proteins of bacterial plasmids and chromosome s, Using immunofluorescence microscopy, we found that KorB, which binds to 12 widely distributed sites on the genome, is located in symmetrically plac ed foci in cells containing IncP-1 plasmids, When maintained by the low-cop y-number P7 replicon, an RK2 segment including incC, korB and the kla, kle and korC regions encodes an efficient partitioning system that gives a patt ern of foci similar to RK2 itself. Symmetrical distribution of KorB foci co rrelates with segregational stability conferred by either the IncP-1 or P7 partitioning systems; KorB distribution follows plasmid distribution. In th e absence of a second partitioning system, incC inactivation resulted in pa ired or clumped foci that were not symmetrically distributed. At a slow gro wth rate, position analysis of foci showed a cycle from one central focus t o two foci (at one- and three-quarter positions) and back, and at a high gr owth rate it showed a cycle from two foci to four and back. This pattern fi ts with the plasmid being coupled to the replication zones in the cell and being moved to successively younger zones by active partitioning, indicatin g a tight association between replication and partitioning.