Identification of the activating region of catabolite gene activator protein (CAP): isolation and characterization of mutants of CAP specifically defective in transcription activation.

We have isolated 21 mutants of catabolite gene activator protein (CAP) defective in transcription activation at the lac promoter but not defective in DNA binding. The amino acid substitutions in the mutants map to a single region of CAP: amino acids 156-162. As assessed in vitro, the substituted CAP variants are nearly completely unable to activate transcription at the lac promoter but bind to DNA with the same affinity and bend DNA to the same extent as wild-type CAP. Our results establish that amino acids 156-162 are critical for transcription activation at the lac promoter but not for DNA binding and DNA bending. In the structure of CAP, amino acids 156-162 are part of a surface loop. We propose that this surface loop makes a direct protein-protein contact with RNA polymerase at the lac promoter.