Purification and properties of the Escherichia coli protein factor required for lambda integrative recombination

A purified preparation of the Escherichia coli integration host factor (IHF) displays two polypeptides of apparent molecular weight 11,000 and 9,500 when analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Under nondenaturing conditions, IHF appears to exist as a 1:1 complex of these two polypeptides. Integrative recombination takes place in vitro when purified IHF and purified Int, a product of a bacteriophage lambda gene, are the only proteins added to reaction mixtures. No recombination is detected in the absence of either protein. The characteristics of the recombination reaction carried out by these two purified proteins are described. Purified IHF binds to DNA; in the presence of Int, a ternary complex is formed at one of the specific recombination sites. IHF hs no detectable endonuclease or topoisomerase activity. Several possibilities for the role of IHF in recombination are considered.