Role of a second catabolite activator protein molecule in controlling initiation of transcription at the galactose operon of Escherichia coli.

The molecular mechanisms whereby RNA polymerase, catabolite activator protein (CAP), and cyclic AMP (cAMP) participate in transcriptional regulation at the galactose operon have been probed by a variety of in vitro techniques. Interactions between purified proteins and promoter-containing DNA fragments were assayed by gel electrophoresis, by resistance to restriction endonuclease digestion, and by monitoring runoff transcripts. The data bear on the multiple functions that CAP performs in gal control. A CAP-cAMP complex can exclude RNA polymerase from one of the two overlapping promoter regions (P2), thereby targeting the enzyme to the other (P1); this process is markedly influenced by the cAMP level. In addition, a second CAP molecule is involved in a cooperative process, which, at low cAMP, is required for efficient formation of transcriptionally competent complexes at P1. This second CAP may serve to stabilize the 1:1:1 CAP-polymerase-gal DNA intermediate under physiological conditions, thus enhancing initiation from P1 relative to P2. Kinetic analysis reveals that the modest effect of CAP on the rate of P1 open complex formation can be resolved into about a 4-fold increase in the binding of RNA polymerase to the P1 region, plus a 1.5-fold elevation in the rate of isomerization of enzyme-promoter complexes to the open state.