16.4: Prokaryotic Gene Regulation - Catabolite Activator Protein (CAP)- An Activator Regulator
- Explain how an activator works to increase transcription of a gene
Just as the trp operon is negatively regulated by tryptophan molecules, there are proteins that bind to the operator sequences that act as a positive regulator to turn genes on and activate them. For example, when glucose is scarce, E. coli bacteria can turn to other sugar sources for fuel. To do this, new genes to process these alternate genes must be transcribed. This type of process can be seen in the lac operon which is turned on in the presence of lactose and absence of glucose.
When glucose levels drop, cyclic AMP (cAMP) begins to accumulate in the cell. The cAMP molecule is a signaling molecule that is involved in glucose and energy metabolism in E. coli . When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein (CAP), a protein that binds to the promoters of operons that control the processing of alternative sugars, such as the lac operon. The CAP assists in production in the absence of glucose. CAP is a transcriptional activator that exists as a homodimer in solution, with each subunit comprising a ligand-binding domain at the N-terminus, which is also responsible for the dimerization of the protein and a DNA-binding domain at the C-terminus. Two cAMP molecules bind dimeric CAP with negative cooperativity and function as allosteric effectors by increasing the protein’s affinity for DNA. CAP has a characteristic helix-turn-helix structure that allows it to bind to successive major grooves on DNA. This opens up the DNA molecule, allowing RNA polymerase to bind and transcribe the genes involved in lactose catabolism. When cAMP binds to CAP, the complex binds to the promoter region of the genes that are needed to use the alternate sugar sources. In these operons, a CAP-binding site is located upstream of the RNA-polymerase-binding site in the promoter. This increases the binding ability of RNA polymerase to the promoter region and the transcription of the genes. As cAMP-CAP is required for transcription of the lac operon, this requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose.
Key Points
- Catabolite activator protein (CAP) must bind to cAMP to activate transcription of the lac operon by RNA polymerase.
- CAP is a transcriptional activator with a ligand-binding domain at the N-terminus and a DNA -binding domain at the C-terminus.
- cAMP molecules bind to CAP and function as allosteric effectors by increasing CAP’s affinity to DNA.
Key Terms
- RNA polymerase : a DNA-dependent RNA polymerase, an enzyme, that produces RNA
- operon : a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together
- promoter : the section of DNA that controls the initiation of RNA transcription