Site-specific recombination of bacteriophage P22 does not require integration host factor

Site-specific recombination by phages lambda and P22 is carried out by mult iprotein-DNA complexes. Integration host factor (IHF) facilitates lambda si te-specific recombination by inducing DNA bends necessary to form an active recombinogenic complex. Mutants tacking IHF are over 1,000-fold less profi cient in supporting lambda site-specific recombination than wild-type cells .;Although the attP region of P22 contains strong IHF binding sites, in viv o measurements of integration and excision frequencies showed that infectin g P22 phages can perform site-specific recombination to its maximum efficie ncy in the absence of IHF. In addition, a plasmid integration assay showed that integrative recombination occurs equally weil in wild-type and ihfA mu tant cells. P22 integrative recombination is also efficient in Escherichia coli in the absence of functional IHF, These results suggest that nucleopro tein structures proficient for recombination can form in the absence of IHF or that another factor(s) can substitute far IHF in the formation of compl exes.