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.