The Binding of Catabolite Activator Protein and RNA Polymerase to the Escherichia Coli Galactose and Lactose Promoters Probed by Alkylation Interference Studies

The Escherichia coli galactose and lactose promoter regions have been studied by alkylation interference experiments. The data reveal those bases or phosphate groups which, when modified, prevent the binding of the catabolite activator protein (CAP) or RNA polymerase and hence are presumably in contact with the proteins. Interference contacts made by CAP at its primary binding sites at gal and lac are quite similar, indicating that CAP-cAMP uses the same mode of binding at these two operons. RNA polymerase, when bound in the presence of CAP-cAMP, exhibits contacts at the gal and lac P1-10 regions very much like those of the lac UV5 and T7 A3 promoters (Siebenlist, U., Simpson, R. B., and Gilbert, W. (1980) Cell 20, 269-281). CAP, therefore, does not detectably alter the structure of the open complex. The binding sites for CAP and RNA polymerase at lac, as deduced from interference experiments, do not overlap. However, at gal a CAP molecule is found much closer to the enzyme, and there is competition for a set of mutual contacts. These experiments thus reveal both similarities and differences in the mechanisms whereby CAP activates transcription at catabolite-sensitive operons.