transcription of the lac operon takes place. Low concentrations of glucose stimulate high levels of cAMP, resulting in increased cAMP-CAP binding to DNA. This increase enhances the binding of RNA polymerase to the promoter and increases transcription of the lac genes by some 50-fold.

The catabolite activator protein exerts positive control in more than 20 operons of E. coli. The response to

CAP varies among these promoters; some operons are activated by low levels of CAP, whereas others require high levels. CAP contains a helix-turn-helix DNA-binding motif and, when it binds at the CAP site, it causes the DNA helix to bend (Figure 16.13). The bent helix enables CAP to interact directly with the RNA polymerase enzyme bound to the promoter and facilitate the initiation of transcription.

When glucose is low

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