NMR study of the structural changes induced in the E. coli lac promoter by the specific binding of the CAP protein.

We have studied the binding of the CAP protein to an 18 base pair lac promoter sequence comprising the core of the CAP recognition sequence. Specific binding of this sequence was established by competition binding assays and comparison of the relative affinities of a number of lac promoter, lac operator, and unspecific sequences of different lengths. The effect of the binding of CAP to the 18 base pair promoter sequence and, for comparison, to an 18 base pair symmetric operator and an oligonucleotide of unrelated sequence have been studied by 1H NMR. Binding of CAP does not bring about any changes in the chemical shift values of the imino proton resonances of the DNA, but causes the selective line broadening of two of the resonances. The comparison of these data with results of gel retardation assays published previously (1) allows the identification and localization of a kink induced in the DNA by the CAP binding to its specific site on the lac promoter.