[Specific association of the CRP-cyclic AMP complex with the control region of the lactose operon of Escherichia coli K 12: a direct fluorimetric study using DNA fragments of different lengths].

Specific-site binding of the cAMP . CRP complex to the control region of the lactose operon of E. coli was measured directly. All of the protein molecules did bind specifically, and the binding constant for the major CRP site was not dependent on the length (62, 219 or 301 base pairs) of the DNA fragments used. Comparing the values of the binding constant measured for the major site and for the weaker "operator" CRP site, and referring to the published "consensus sequence" derived from the known CRP sites in a series of operons, we suggest that two sub-sites support additive contribution to the total binding free energy.