Multiple Choice
How does extracellular glucose inhibit transcription of the lac operon?
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16. Regulation of Expression
Glucose's Impact on Lac Operon
Multiple Choice
When glucose is present:
A
cAMP is high, CRP binds to the activator binding site, and transcription of the lac operon is turned off.
B
cAMP is low, CRP binds to the site activator binding site, and transcription of the lac operon is turned on.
C
cAMP is high, CRP does not bind to the activator binding site, and transcription of the lac operon is turned on.
D
cAMP is low, CRP does not bind to the activator binding site, and transcription of the lac operon is turned off.
Verified step by step guidance1
Understand the role of cAMP and CRP in the regulation of the lac operon. cAMP (cyclic AMP) is a signaling molecule that, when bound to CRP (cAMP receptor protein), facilitates the binding of CRP to the activator binding site, enhancing transcription of the lac operon.
Recognize that the presence of glucose affects the levels of cAMP. When glucose is abundant, the levels of cAMP are low because glucose inhibits the enzyme adenylate cyclase, which is responsible for converting ATP to cAMP.
Analyze the relationship between cAMP levels and CRP binding. Low cAMP levels mean that CRP cannot bind to the activator binding site because cAMP is required for CRP to change its conformation and bind effectively.
Consider the effect on the lac operon transcription. Without CRP binding to the activator site, the lac operon is not activated, and transcription is turned off. This is because CRP-cAMP complex is necessary for RNA polymerase to efficiently bind to the promoter of the lac operon.
Conclude that when glucose is present, cAMP is low, CRP does not bind to the activator binding site, and transcription of the lac operon is turned off. This is consistent with the cell's preference to use glucose over lactose when both are available.
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