CAP-cAMP

A Barnes Ice Cap B Devon Ice Cap Agassiz Ice Cap Camp Century GISP2 GRIP Renland... 

The interaction between the CAP-cAMP complex and DNA is shown in Fig. 4. Each subunit of the dimeric inducer (yellow or orange) binds one molecule of cAMP (red). Contact with the DNA (blue) is mediated by two recognition helices that interact with the major groove of the DNA. The bending of the DNA strand caused by CAP has functional significance. 

Cover design Cycius, Stuttgart Cover drawing CAP cAMP bound to DNA Typesetting by primustype Hurier GmbH, Notzingen... 

The CAP binds to DNA with the consensus sequence 5 -A A ATGTG ATCT/5 -AGATCACATTT, which may be located at variable distances from the promoter.133 How does binding of the CAP-cAMP complex increase the rate of initiation of mRNA transcription The anwer evidently lies in direct interaction between CAP and the N-terminal domain of the RNAP a subunit.54d 129 Binding of CAP induces a 90° bend in the DNA, which may facilitate the protein-protein interaction and may lead to looping.130134... 

Control by looping. The arabinose utilization operon of E. coli, araBAD, encodes proteins needed for uptake of arabinose and conversion to D-xylulose 5-P. The repressor AraC in the absence of arabinose binds at operator 1 (Oj) to prevent further synthesis of repressor (autorepression) and also at the aral region to repress transcription of operon araBAD. The operator 2 (02) site, which is 211 bp upstream from aral, is also needed for this repression.145-14713 A loop is apparently formed by repressor binding (Fig. 28-7). Binding of arabinose to the repressor converts it into an activator, which stimulates initiation of transcription at the BAD promoter. Further stimulation is provided by the CAP-cAMP complex, which binds next at aral. 

When lac binds the inducer, it loses its affinity for the operator and dissociates. RNA polymerase is now free to transcribe lacZ, lacY, and lacA. If glucose is not present, the CAP-cAMP complex will form and bind the CAP site. This CAP-cAMP complex bound at the CAP site enhances transcription of the operon. 

Although the inducer causes the lacl repressor protein to dissociate from the operator, the glucose will lead to a low level of cAMP (low adenylate cyclase activity). As the CAP-cAMP complex dissociates, the CAP protein will leave the CAP site. This will repress transcription of lacZ, lac Y, and lac A (catabolite repression). 

The CAP-cAMP complex stimulates the initiation of transcription by approximately a factor of 50. A major factor in this stimulation is the recruitment of RNA polymerase to promoters to which CAP is bound. Studies have been undertaken to localize the surfaces on CAP and on the a subunit of RNA polymerase that participate in these interactions (Figure 3111). These energetically favorable protein-protein contacts increase the likelihood that transcription will be initiated at sites to which the CAP-cAMP complex is bound. Thus, in regard to the lac operon, gene expression is maximal when the binding of allolactose relieves the inhibition by the lac repressor, and the CAP-cAMP complex stimulates the binding of RNA polymerase. 

As arabinose levels drop owing to its metabolism by the araBAD-encoded proteins, AraC is converted back to a repressor and araBAD expression is decreased, even though CAP-cAMP complexes are still present in the cell. Interestingly, when glucose and arabinose are both abundant, the ara operon is repressed and, although the mechanistic basis for this is not totally understood, this result supports the model that araBAD induction is dependent on both AraC-arabinose and CAP-cAMP complexes. 

The ara operon is positively controlled by the activity of AraC, which can act both as a repressor and activator of ara operon transcription, depending on the intracellular concentrations of arabinose and glucose. In the absence of arabinose, AraC is a repressor and binds to two separate operator sites in the ara operon, leading to the formation of an inhibitory DNA loop. However, in the presence of arabinose, which binds to AraC, and in the presence of CAP-cAMP complexes, the inhibitory DNA loop is disrupted and CAP-cAMP complexes are able to stimulate transcription of the ara operon. 

Q The control sites of the facoperon. The CAP-cAMP complex, not CAP alone, binds to the CAP site of the lac promoter. When the CAP site on the promoter is not occupied, RNA polymerase does not bind. 

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