Participation of Escherichia coli integration host factor in the P1 plasmid partition system

Stable maintenance of the plasmid prophage of bacteriophage P1 requires the P1 ParB protein, which acts on a DNA site termed parS. Fractionation of extracts from Escherichia coli cells overproducing ParB revealed that a host factor, in addition to ParB, is required to observe maximal binding to parS, as detected by a nitrocellulose filter retention assay. Two observations indicated that this factor is E. coli integration host factor (IHF): purified IHF substituted specifically for host factor from a crude lysate, and lysates prepared from cells deficient in the beta subunit of IHF (E. coli hip mutants; also called himD) contained no host factor activity. Binding studies in vitro and competition experiments in vivo suggest that two types of ParB-parS DNA complexes can exist that differ in (i) the presence of IHF, (ii) the amount of parS sequence with which the proteins interact, and (iii) the specificity of their participation in partition. Under normal conditions, with the intact P1 partition region and wild-type bacteria, P1 plasmids apparently use IHF to assist ParB in the assembly of a functional partition complex at parS.