Identification of the Target of a Transcription Activator Protein by Protein-Protein Photocrosslinking

Here it is shown, with the use of protein-protein photocrosslinking, that the carboxyl-terminal region of the alpha subunit of RNA polymerase (RNAP) is in direct physical proximity to the activating region of the catabolite gene activator protein (CAP) in the ternary complex of the lac promoter, RNAP, and CAP. These results strongly support the proposal that transcription activation by CAP involves protein-protein contact between the carboxyl-terminal region of the alpha subunit and the activating region of CAP.