Catabolite activator protein-induced DNA bending in transcription initiation

We describe experiments that enable us to track the presence and direction of the DNA bend induced by Escherichia coli catabolite activator protein (CAP) through the intermediate stages of transcription initiation at the lac promoter. Transcriptional complexes examined were formed on superhelical templates to enhance specific complex formation, and detected by electrophoretic analysis after restriction digestion. We found that the bend is maintained and even increased upon formation of closed and open complexes. Our results exclude the hypothesis that the energy of the CAP-induced bend is used to promote open complex formation. We now suggest a new model, in which DNA wraps around the CAP-polymerase complex to form a writhing structure equivalent to that at the end of an interwound superhelical domain. Formation of this structure may facilitate open complex formation. We further propose that the stored bend energy may be used to help counteract strong protein-protein or protein-DNA interactions, thus assisting the process of RNA polymerase escape from the promoter.