Catabolite activator protein CAP

Regulatory proteins Insulin Somatotropin Thyrotropin lac repressor NEl (nuclear factor 1) Catabolite activator protein (CAP) API... 

In cyclic nucleotide-regulated channels, this domain serves as a high-affinity binding site for 3-5 cyclic monophosphates. The CNBD of channels has a significant sequence similarity to the CNBD of most other classes of eukaryotic cyclic nucleotide receptors and to the CNBD of the prokaryotic catabolite activator protein (CAP). The primary sequence of CNBDs consists of approximately 120 amino acid residues forming three a-helices (oA-aC) and eight (3-strands ( 31- 38). 

Busby S, Ebright RFI Transcription activation by catabolite activator protein (CAP). J Mol Biol 1999 293 199. 

When the allosteric catabolite activator protein (CAP or CRP) binds cAMP, the dimeric protein is able to bind to a site in the promoter and to increase the rate of initiation of transcription... 

The organism senses whether glucose is available by another regulatory mechanism which cooperates with the lac repressor and the lac operator. The promoter is therefore sub-divided into two specific regions, each of distinctive function. One is the RNA polymerase entry site, where RNA polymerase first becomes bound to DNA (cf. DNA transcription, Appendix 5.6), and the other is the protein binding site for the catabolite activator protein (CAP) (Fig. 5.39). The CAP protein binding site controls the polymerase site which, when bound to the DNA, allows successful transcription provided that the repressor is not bound. When the CAP protein is not bound, then RNA polymerase cannot bind and transcription cannot take place. 

Catabolite activator protein, CAP (also called cAMP receptor protein, CRP) is an activator required for high level transcription of the lac operon. The active molecule is a CRP dimer that binds 3 5 cyclic AMP to form a CRP-cAMP complex. CRP-cAMP binds to the lac promoter and increases the binding of RNA polymerase, stimulating transcription of the lac operon. CRP dimer without cAMP cannot bind to this DNA. The action of CRP depends upon the carbon source available to the bacterium. When glucose is present, the intracellular level of cAMP falls, CRP cannot bind to the lac promoter and the lac operon is only weakly transcribed. When glucose is absent, the level of intracellular cAMP rises, the CRP-cAMP complex stimulates transcription of the lac operon and allows lactose to be used as an alternative carbon source. 

There is also an aspect of positive control in the lac operon. The catabolite activator protein (CAP), carrying bound cAMP, is required for the binding of RNA polymerase to the promoter i.e., it has a direct, positive effect on transcription. However, relief of repression (i.e., induction) will not occur in the presence of glucose, because glucose lowers the level of cAMP, so that CAP is unable to exert its effect. This reflects the preference of the cell to use glucose rather than lactose as a carbon source. Thus it can be seen that the cell stringently controls expression of the lac genes it expresses them only if it needs to metabolize lactose. 

All the DNA-binding proteins discussed thus far function by inhibiting transcription until some environmental condition, such as the presence of lactose, is met. There are also DNA-binding proteins that stimulate transcription. One particularly well studied example is the catabolite activator protein (CAP), which is also known as the cAMP response protein (CRP). When bound to cAMP, CAP, which also is a sequence-specific DNA-binding protein, stimulates the transcription of lactose- and arabinose-catabolizing genes. Within the lac operon, CAP binds to an inverted repeat that is centered near position -61 relative to the start site for transcription (Figure 31.10). CAP functions as a dimer of identical subunits. 

Figure 31.10. Binding Site for Catabolite Activator Protein (CAP). This protein binds as a dimer to an inverted repeat that is at the position -61 relative to the start site of transcription. The CAP binding site on DNA is adjacent to the position at which RNA polymerase hinds. 

The answer is d. (Murray, pp 468-487. Scriver, pp 3-45. Sack, pp 245-257. Wilson, pp 151-180.) Several operons in E. coli, including the lac operon, are subject to catabolite repression. In the presence of glucose, there is decreased manufacture of cyclic AMP (cAMP) by adenylate cyclase. Low glucose levels increase production of cAMP, which binds to the catabolite activator protein (CAP). The cAMP-CAP complex binds to the promoters of several responsive operons at catabolite activator protein... 

Figure 4.13. Crystal structure of the catabolite activator protein (CAP) with DNA, downloaded from the Protein Data Bank (ID 1CGP) and visualized in RasMol [113]. The DNA is red and yellow, the protein green and blue. (See color plates).

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