Q.13 What will be the binding status of regulatory proteins in lac operon when concentrations of both
lactose and glucose are very low in the culture medium?
(A) Only the repressor remains bound to the operator
(B) Only the cyclic AMP-Catabolic Activator Protein (cAMP-CAP) complex remains bound to the
CAP binding site
(C) Neither the repressor nor cAMP-CAP complex remain bound to their respective binding sites
(D) Both the repressor and cAMP-CAP complex remain bound to their respective binding sites
Lac Operon Binding Status: Low Lactose and Low Glucose
When both lactose and glucose concentrations are very low in the culture medium, the lac repressor binds to the operator, blocking transcription, while low glucose raises cAMP levels but the absence of lactose prevents full operon activation.
Correct Answer
The correct choice is (A) Only the repressor remains bound to the operator.
Detailed Mechanism
Low lactose means no allolactose forms to inactivate the lac repressor, so it stays bound to the operator, preventing RNA polymerase access. Low glucose increases cAMP, allowing cAMP-CAP to bind its site upstream of the promoter, but without lactose-induced derepression, this activator cannot stimulate transcription effectively. Thus, repression dominates, ensuring no wasteful lac enzyme production when neither sugar is available.
Option Analysis
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(A) Only the repressor remains bound to the operator: Correct. No lactose keeps repressor active on operator; cAMP-CAP binds but transcription stays off due to blockage.
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(B) Only the cyclic AMP-Catabolic Activator Protein (cAMP-CAP) complex remains bound to the CAP binding site: Incorrect. cAMP-CAP does bind (high cAMP from low glucose), but repressor also binds operator, overriding activation.
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(C) Neither the repressor nor cAMP-CAP complex remain bound to their respective binding sites: Incorrect. Repressor binds firmly without lactose; cAMP-CAP binds due to high cAMP.
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(D) Both the repressor and cAMP-CAP complex remain bound to their respective binding sites: Partially true on binding but misleading—repressor blocks despite cAMP-CAP presence, so operon stays repressed.
The lac operon binding status low lactose low glucose scenario highlights bacterial gene regulation efficiency in E. coli. When both sugars are scarce, the system prioritizes energy conservation through precise protein-DNA interactions.
Core Regulatory Elements
The lac operon features:
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Lac repressor: Binds operator without lactose, blocking lacZYA genes for β-galactosidase, permease, and transacetylase.
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cAMP-CAP complex: Binds CAP site when glucose is low (high cAMP), aiding RNA polymerase but requiring operator clearance.
Low lactose prevents allolactose formation, keeping repressor bound; low glucose enables cAMP-CAP binding.
Binding States Table
| Condition | Repressor (Operator) | cAMP-CAP (CAP Site) | Transcription Level |
|---|---|---|---|
| Low Lactose, Low Glucose | Bound | Bound | Off/None |
| High Lactose, Low Glucose | Unbound | Bound | High |
| Low Lactose, High Glucose | Bound | Unbound | Off |
| High Lactose, High Glucose | Unbound | Unbound | Low/Basal |
This table summarizes lac operon regulatory proteins binding status across sugar levels.
Implications for Molecular Biology
In lac operon binding status low lactose low glucose, repression prevents unnecessary enzyme synthesis, exemplifying negative control dominance over positive activation. This dual regulation—repressor for lactose response, CAP for glucose catabolite repression—optimizes resource use, relevant for biotech fermentation and genetic engineering studies. Students solving MCQs benefit from visualizing these states for exams in genetics or microbiology.
