Integration host factor (IHF) is a small, basic protein that is needed for efficient recombination of bacteriophage lambda, as well as for other host and viral functions. We have constructed strains in which the two subunits of IHF, encoded by the himA and hip genes of Escherichia coli, are expressed under the control of the lambda rho L promoter. Separate overexpression of himA and hip led to the production of unstable and insoluble peptides, respectively. In contrast, the overexpression of both genes conjointly led to the accumulation of large amounts of active IHF. Extracts of such cells provided the starting material for a rapid purification procedure that results in milligram quantities of apparently homogeneous IHF.

The N gene product of coliphage lambda acts with host factors (Nus) through sites (nut) to render subsequent downstream transcription resistant to a variety of termination signals. These sites, nutR and nutL, are downstream, respectively, from the early promoters PR and PL. Thus a complicated set of molecular interactions are likely to occur at the nut sites. We have selected mutations in the nutR region that reduce the effectiveness of pN in altering transcription initiating at the PR promoter. DNA sequence analysis of three independently selected mutations revealed, in each case, a deletion of a single base pair in the cro gene. Consideration of the effect of such mutations on the extension of translation of cro message into the adjacent downstream nut region led to the identification of a consensus sequence CGCTCT(T)TAA that appears to play a role in the recognition of a host factor, possibly the NusA protein.

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