The extracellular glucose inhibit transcription of the lac operon (D) by reducing the levels of intracellular cAMP.
Lac operon is the assembly of various genes that are involved in the uptake and metabolism of lactose of E. coli or any other bacteria. It consists of a regulator gene, promoter gene, operator and structural gene. Structural genes are three: z, y and a. Each codes for a different enzyme.
cAMP is the cyclic Adenosine Monophosphate. It is produced by the bacteria when there are low levels of glucose in it. Hence it is also named as hunger signals. Therefore, cAMP is responsible for activating the operon to produce lactose.
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