The integration host factor (IHF) of Escherichia coli is a small, sequence-specific DNAbinding protein. The specific and nonspecific binding constants of IHF were estimated by gel-retardation assays. The equilibrium association constant of IHF for the H’ site in λattP is 6.8 x 108 M-1 (Kd = 1.5 nM), and the nonspecific binding constant is 5.8 x 105 M-1 (Kd = 1.7µM), giving a selectivity of approximately 1000-fold for a specific site over random sequences. To study the molecular determinants specifying IHF binding, we used a series of 41 oligonucleotides containing adenine analogues that modified the surfaces of the major and minor grooves of the DNA. Many of the analogue substitutions within the previously defined consensus region caused decreased binding. Replacement with various analogues outside the consensus domain had little effect. Quantifying the binding constants for those sites with reduced affinities indicated an interaction with the minor groove within the consensus sequence. The binding constants of sites with 2-aminopurine and an inosine substitution within the same region suggest that IHF may also interact with the major groove. Thus, the specific interaction of IHF with its H’ site likely involves interactions with both the minor and major grooves of the DNA.
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