The Escherichia coli catabolite activator protein (CAP) activates transcription at P _lac , P _gal , and other promoters through interactions with the RNA polymerase α subunit carboxyl-terminal domain (αCTD). We determined the crystal structure of the CAP-αCTD-DNA complex at a resolution of 3.1 angstroms. CAP makes direct protein-protein interactions with αCTD, and αCTD makes direct protein-DNA interactions with the DNA segment adjacent to the DNA site for CAP. There are no large-scale conformational changes in CAP and αCTD, and the interface between CAP and αCTD is small. These findings are consistent with the proposal that activation involves a simple “recruitment” mechanism.