Co-operative interactions between the catabolite gene activator protein and the lac repressor at the lactose promoter

The catabolite gene activator protein (CAP) and the lac repressor regulate the transcriptional activity of the lactose operon. An early step in the regulatory functions of these proteins is their binding to specific DNA sequences within the lac promoter-operator region. Using the gel electrophoresis mobility-shift technique, we have found that the ternary complex with CAP and repressor bound to their respective highest affinity sites is 4 to 11-fold more stable than is predicted from the affinities of the independently bound proteins. This favorable binding interaction is unexpected, because CAP and lac repressor exert opposing effects on lac operon transcription. Deoxyribonuclease I footprinting analyses show that interacting proteins remain bound to the sites occupied when the proteins bind singly. These sites have a center-to-center separation of 72 base-pairs (corresponding to 6.9 turns of a B-form DNA helix), and thus occupy the same "face" of the DNA cylinder. Such an orientation is compatible with models of the ternary complex in which DNA curvature facilitates the interaction of CAP and lac repressor.