MUTANTS OF ESCHERICHIA-COLI INTEGRATION HOST FACTOR - DNA-BINDING ANDRECOMBINATION PROPERTIES

Integration host factor (IHF) is a protein encoded by Escherichia coli , which was first discovered as a requirement for bacteriophage lambda site-specific recombination. In this study, we characterized mutants of IHF for their ability to bind to various IHF binding sites in vivo and to promote recombination of lambda in vitro. DNA-binding in vivo w as monitored using the challenge-phage system. If IHF binds to its DNA -binding site that has been placed into the P-ant region of bacterioph age P22, it acts as a repressor of the ant (antirepressor) gene, leadi ng to the formation of lysogens of Salmonella typhimurium. If IHF cann ot bind to its site, antirepressor is made leading to cell lysis. Chal lenge phages containing chimeras of different lambda IHF binding sites were constructed to test the contribution to the binding of a dA+dT-r ich region, found in the sequence of the H' site but not in the H1 sit e. In one case, the binding of mutant IHF proteins was enhanced by the presence of the dA+dT-rich region, indicating that IHF may be affecte d by neighboring bases and local DNA structure when it binds to its si te. A subset of the mutant proteins retained the ability to form a loo ped attL complex in vivo, representing part of a higher-order protein- DNA complex (the 'intasome'). Additionally, this same subset of protei ns also promoted the integration and excision of bacteriophage lambda in vitro. Thus, these mutant proteins not only retain their DNA-bendin g ability but make any protein-protein contacts necessary to form a re combination-proficient intasome.