The bacteriophage lambda transcriptional activator protein, cII, coordinately regulates transcription from two phage promoters that control lysogenic development. We demonstrate that cII is a DNA binding protein that selectively interacts with a repeat sequence in the -35 region of the promoter. Furthermore, cII is shown to bind mainly one face of the DNA helix and to make its contacts primarily in the major groove of the DNA. RNA polymerase sees this same region from the opposite side and sandwiches the DNA helix between itself and cII.