Incell biology,catabolite activator protein (CAP), which is also known ascAMP receptor protein (CRP), is atrans-actingtranscriptional activator inbacteria that effectivelycatalyzes the initiation ofDNAtranscription by interacting withRNA polymerase in a way that causes the DNA to bend.[1]
CAP's name reflects the protein's ability to affect transcription of genes involved in manycatabolic pathways. For example, when the amount ofglucose transported into a cell is low, a cascade of events results in the increase of cAMP levels in the cell'scytosol, and this increase in cAMP levels is sensed by CAP, which goes on to activate the transcription of many other catabolic genes.
CAP exists as ahomodimer in solution, and it is bound to by twocyclic AMP (cAMP)ligand molecules with negativecooperativity. By increasing CAP'saffinity forDNA, cyclic AMP functions as anallosteric effector.
With its cyclic-AMP ligand, CAP binds a DNA region upstream from the site at which RNA polymerase binds and activates transcription through protein-protein interactions with RNA polymerase's α-subunit. This protein-protein interaction both catalyzes the formation of the RNAP-promoter closed complex andisomerizes the RNAP-promoter complex to the open conformation.
CAP has a characteristichelix-turn-helix motif structure that allows it to bind to successivemajor grooves on DNA. The two helices are reinforcing, each causing a 43° turn in the structure, with an overall 94° degree turn in the DNA.[2] Each subunit of CAP is composed of aligand-binding domain at theN-terminus (CAPN, residues 1–138) and aDNA-binding domain at theC-terminus (DBD, residues 139–209).[3][4]
One example of the role of CAP is its criticality in activating the ability ofE. coli to metabolizelactose.The cAMP-CAP complex allows RNA polymerase to bind to thelac operon and transcribe its genes, which encode the proteins required for breaking down lactose into glucose andgalactose.[3][4] Regulation of thelac operon is vital forE. coli because glucose is more easily metabolized—and is therefore a more economical source ofcarbon—than lactose. So the cell "prefers" glucose, and its presence causes thelac operon to be repressed regardless how much lactose may be available.
Such conditional turning off of genes for metabolizing less-preferred substances, known ascatabolite repression, is common in bacteria, and CAP plays an important role in it. Besides its function in activating thelac operon, CAP has a similar role in theMal regulon,[5] controlling the expression ofmalT, a gene critical in the uptake and metabolism ofmaltodextrins.[6]
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