PARTITION OF P1 PLASMIDS IN ESCHERICHIA-COLI MUKB CHROMOSOMAL PARTITION MUTANTS

The partition system of the low-copy-number plasmid/prophage of bacter iophage P1 encodes two proteins, ParA and ParB, and contains a DNA sit e called parS. ParB and the Escherichia coli protein IHF bind to parS to form the partition complex, in which parS is wrapped around ParB an d IHF in a precise three-dimensional conformation. Partition can be th ought of as a positioning reaction; the plasmid-encoded components ens ure that at least one copy of the plasmid is positioned within each ne w daughter cell. We have used an E. coli chromosomal partition mutant to test whether this positioning is mediated by direct plasmid-chromos omal attachment, for example, by pairing of the partition complex that forms at parS with a bacterial attachment site. The E. coli MukB prot ein is required for proper chromosomal positioning, so that mukB mutan ts generate some cells without chromosomes (anucleate cells) at each c ell division. We analyzed the plasmid distribution in nucleate and anu cleate mukB cells. We found that P1 plasmids are stable in mukB mutant s and that they partition into both nucleate and anucleate cells. This indicates that the P1 partition complex is not used to pair plasmids with the host chromosome and that P1 plasmids must be responsible for their own proper cellular localization, presumably through host-plasmi d protein-protein interactions.