Catabolite gene activator protein complex with DNA
Escherichia coli (bacteria)
The Escherichia coli catabolite gene activator protein (CAP) (cAMP receptor protein CRP), which regulates transcription of several genes in Escherichia coli and related bacteria, is a transcriptional activation factor. The binding of cAMP to CAP induces CAP to bind the specific DNA sequence located in the promoter region. The complex of CAP-cAMP and DNA recruits an RNA polymerase, and the RNA polymerase binds to the promoter region and initiates a transcription. Thus, CAP activates a transcription.
The three-dimensional structure of CAP complexed with DNA revealed the mechanisms of transcription activation. When cAMP binds to the CAP homodimer, activated CAP is produced and this activated CAP binds to DNA, inducing a 90 degree bend in the DNA. In protein-DNA binding, the protein secondary structure "helix-turn-helix" motif is used. CAP functions as a homodimer. The N- and C-terminal domains of CAP bind to cAMP and DNA, respectively, and both N- and C-terminal domains contain RNA polymerase-interaction sites. When the signal of a transcription initiation is given (e.g. when the concentrations of glucoses decrease, the signals of transcription initiations for lactases are given.), cAMP binds to CAP in order to transmit this signal, and activates CAP. The activated complex, CAP-cAMP, recognizes and binds to the specific DNA sequence located in the promoter region of the operon. This triggers the 90 degree bend in the DNA. The large bend is the collective result of two 40 degree kinks in the DNA, induced by the interactions between CAP-cAMP and phosphates of the strongly conserved DNA sequence TGTGA. In addition, sequence specificity is achieved through hydrogen-bond interactions between three side chains of the CAP recognition helix and three base pairs in the DNA major groove. As a result of the specific interaction with DNA, CAP makes contacts with RNA polymerase and activates transcription of the operon by RNA polymerase.
Protein Data Bank (PDB)
Schultz, S.C. Shields, G.C. Steitz, T.A.; "Crystal structure of a CAP-DNA complex: the DNA is bent by 90 degrees."; Science; (1991) 253:1001-1007 PubMed:1653449.
author: Yuko Tsuchiya