If the binding of the CAP-CAMP complex promotes transcription of the induced lac

Question

If the binding of the CAP-CAMP complex promotes transcription of the induced lac operon, then how does the negative control of the lac operon by glucose function through the CAP-CAMP complex? See Section 18.3 (D e 373 O When glucose concentration is high, the concentration of CAMP and the complex are low. O When glucose concentration is high, the concentration of CAMP and the complex are high. O When glucose concentration is high, the concentration of CAP and the complex are high. When glucose concentration is high, the concentration ofCAP and the complex are high. Submit Hints My Answers Give Up Review Part

Explanation / Answer

cAMP is synthesised by the enzyme adenylate cyclase and its concentration is related to the concentration of glucose.So when the level of glucose in the cell is high, the cAMP concentration is low and vice-versa.Like in eukaryotic cells,cAMP binds to a protein CRP(cAMP receptor protein) in prokaryotic cells. CRP and cAMP bind together to form a complex (cAMP-CRP) which regulates the lac system.The  cAMP-CRP complex must bind to the activation site in the promoter region in order for the transcription of the lac-operon to occur.

In absence of glucose, cAMP-CRP binds near lac promoter site and increases the transcription by 50 fold as the CRP directly interacts with alpha subunit of RNA polymrerase.However, in the presence of glucose, cAMP synthesis decreases, which in turn lowers the expression of the lac operon even when lactose (normal inducer) is present.The ability of the glucose to repress the synthesis of enzymes required for the metabolism of other sugars, is referred to as glucose effect or catabolite repression. When E.coli encounters both lactose and glucose in medium,it prefers to metabolise the glucose first as lactose metabolism requires extra enzymes.

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