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Concept 12: The Effect of Lactose in the Absence of Glucose on the lac Operon

In the presence of lactose and absence of glucose, cyclic AMP (cAMP) joins with a catabolite activator protein that binds to the lac promoter and facilitates the transcription of the lac operon.

In some texts, the catabolite activator protein (CAP) is called the cAMP-receptor protein.


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When the concentration of glucose is low, cAMP accumulates in the cell. The binding of cAMP and the catabolite activator protein to the lac promoter increases transcription by enhancing the binding of RNA polymerase to the lac promoter.