What is CAP? How does CAP work?

The Escherichia coli catabolite gene activator protein (CAP) is a DNA binding protein involved with the transcription of several genes, including those that code for enzymes involved in the metabolism of certain sugars (i. e. lactose, maltose, and arabinose.) Basically, CAP is responsible for the global regulation of carbon utilization. Upon binding cAMP, CAP binds to a conserved DNA sequence from which it can either activate or repress transcription initiation from various promoters. In some cases clusters of several promoters are all controlled by a single cAMP-CAP complex bound to the DNA.

Once CAP has bound cAMP, the protein exhibits a higher affinity for a specific conserved DNA sequence. When the intracellular level of cAMP increases, the second messenger is bound by CAP and the cAMP-CAP complex binds to the DNA. Once bound, it is able to stimulate the transcription of the aforementioned genes. DNA bound by the CAP-cAMP complex is bent by ~90 degrees. This DNA bend, coupled with a protein-protein interaction between CAP and RNA polymerase is thought to be the mechanism by which CAP regluates transcription initiation on the chromosome.