Q74. Which of the following event(s) would contribute to the induction of lac
operon in a wild–type strain of E. coli?
(A) Accumulation of allolactose in the cell
(B) Direct binding of cAMP to the promoter DNA
(C) Binding of cAMP to a specific protein leading to its interaction with the
promoter
(D) Elimination of cAMP from the cell
The correct answer is options (A) and (C).
Lac Operon Basics
The lac operon in wild-type E. coli regulates lactose metabolism genes (lacZ, lacY, lacA) through negative control by the lac repressor and positive control by the cAMP-CAP complex. Induction occurs when lactose is present and glucose is low, allowing efficient transcription.
Option Analysis
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(A) Accumulation of allolactose: Allolactose, formed from lactose by β-galactosidase, binds the lac repressor, causing it to release the operator and permit transcription.
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(B) Direct binding of cAMP to promoter DNA: cAMP does not bind DNA directly; it binds CAP first, forming a complex that interacts with the promoter.
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(C) Binding of cAMP to specific protein leading to promoter interaction: cAMP binds CAP (specific protein), and the cAMP-CAP complex binds upstream of the promoter to activate RNA polymerase.
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(D) Elimination of cAMP: Removing cAMP prevents CAP activation, repressing the operon due to catabolite repression.
Introduction to Lac Operon Induction
Lac operon induction in E. coli enables bacteria to metabolize lactose efficiently when glucose is scarce. This classic gene regulation model involves allolactose as the inducer and cAMP-CRP (CAP) for positive control, crucial for CSIR NET Life Sciences preparation.
Mechanism of Induction
In wild-type E. coli, the lac repressor binds the operator, blocking transcription without lactose. Allolactose accumulation inactivates the repressor, while low glucose raises cAMP levels. cAMP binds CAP, and the complex enhances promoter binding by RNA polymerase for full induction.
Role of Allolactose and cAMP-CAP
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Allolactose binds the repressor tetramer, inducing a conformational change that releases the operator.
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cAMP-CAP binds a site upstream of the promoter, boosting transcription 50-fold when glucose is absent.
This dual regulation ensures energy efficiency.
Common MCQ Insights
Questions on lac operon induction often test allolactose’s role and cAMP’s indirect action via CAP, distinguishing it from direct binding. Elimination of cAMP or glucose presence represses the operon.
