Interaction of the cAMP receptor protein with the lac promoter.

The cyclic AMP receptor protein (CRP) stimulates transcription of the lactose operon by binding to the lac promoter. I have identified those 5-positions of thymines in the promoter that lie close to bound CRP. Although ultraviolet irradiation of DNA with 5-bromouracil substituted in place of thymine normally cleaves the DNA at the bromouracils, a protein bound to the DNA can perturb these cleavages at those locations at which the protein lies close to the bromine. The contacts inferred from this photochemical probe and the results of nucleolytic attack of this complex by exonuclease III support a model where the cyclic AMP receptor protein binds to the promoter making symmetrical contacts with one face of the double helix and then stimulates transcription through contacts with RNA polymerase.