Integration host factor is necessary for lysogenization of Escherichia coli by bacteriophage P2

Whether infection by bacteriophage P2 results in lysogenization of the host or vegetative growth of the phage depends upon a race between transcription from the repressor promoter Pc and the early promoter Pe; transcription from these promoters is mutually exclusive, since the Pc repressor Cox is formed from the Pe transcript and the Pe repressor C from the Pc transcript. The involvement of integration host factor (IHF) in the lysogenization of Escherichia coli K12 by P2 was tested by comparing wild-type and IHF-deficient (himA and himD) mutants. No lysogenic clones were formed following infection of the mutant bacteria. A switch plasmid that contains Pc-C-cat and Pe-cox-kan was used to test the choice for expression of Pc versus Pe. In the wild-type K12 bacteria, 20% of the clones expressed Pe transcription and 80% Pc transcription, whereas all transformed IHF-defective clones expressed transcription from Pe only. The effects of IHF on the in vivo expression of the Pe and Pc promoters were only marginal. The IHF protein was found to bind upstream of the Pe promoter, where a potential ihf sequence is located.