Question

Which of the following condition will favour synthesis of $\beta$-galactosidase in E. coli cells

• A. Growing them in a nutrient medium containing glucose as a C source
• B. Growing them in a nutrient medium containing glycerol as a C source
• C. Growing them in a nutrient medium containing lactose as a C source
• D. Growing them in a nutrient medium containing some antibiotic into it

Hint:

The lac operon is under dual control:
- Negative regulation via the repressor (derepressed by lactose/allolactose).
- Positive regulation via CAP-cAMP (activated by low glucose).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The lac operon in Escherichia coli regulates the transcription of genes involved in lactose metabolism, including $\beta$-galactosidase ($\text{LacZ}$), which hydrolyzes lactose into glucose and galactose.

Step 2: Detailed Explanation:
- Under default conditions when lactose is absent, the lac repressor binds to the operator, blocking transcription of the operon.
- When E. coli is grown in a medium containing lactose as the carbon source (and lacking glucose):
1. Lactose enters the cell and is converted into allolactose, which acts as an inducer.
2. Allolactose binds to the lac repressor, causing a conformational change that prevents it from binding to the operator.
3. This allows RNA polymerase to transcribe the operon, leading to the synthesis of $\beta$-galactosidase.
- If glucose is present (Option A), catabolite repression occurs. This lowers cyclic AMP (cAMP) levels and prevents transcription of the operon even if lactose is present.

Step 3: Final Answer:
Growing the cells in a medium containing lactose as the carbon source induces $\beta$-galactosidase synthesis.