Abstract
Previously, we reported that substitution of Glu-181 of the catabolite gene activator protein (CAP) by lysine, leucine, or valine results in a protein that has specificity for A X T base pairs at positions 7 and 16 of the DNA recognition site, rather than G X C base pairs as is the case with the wild-type CAP. In this paper, we deduce from these genetic data both (i) the specific chemical interactions by which amino acid side chains at position 181 interact with base pairs 7 and 16 and (ii) the precise alignment between the structures of the CAP and DNA in the intermolecular CAP-DNA complex. Our analysis supports the idea that the two symmetry-related F alpha-helices of the CAP dimer interact with successive major grooves of right-handed B-type DNA [Pabo, C. & Lewis, M. (1982) Nature (London) 298, 443-447; and Steitz, T., Weber, I. & Matthew, J. (1983) Cold Spring Harbor Symp. Quant. Biol. 47, 419-426].