75. An E. coli strain is grown initially on glucose as the sole carbon source. Upon complete
consumption of glucose following 12 h of growth, lactose is added as the sole carbon source and
the strain is further grown for 12 h . Assuming that the E. coli strain has a functional wild type lac
operon, which one of the following profiles is the most ACCURATE representation of 𝛽
galactosidase ( 𝛽 − 𝑔𝑎𝑙 ) expression (in arbitrary units)?
(A) i
(B) iii
(C) ii
(D) iv

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Question Overview

An E. coli strain is initially grown on glucose as the sole carbon source for 12 hours. After glucose is completely consumed, lactose is added as the sole carbon source, and growth continues for another 12 hours.

Assuming the strain has a functional wild-type lac operon, which graph most accurately represents β-galactosidase (β-gal) expression over time?

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Key Biological Concepts Involved

To answer this, we must understand lac operon regulation:

🔹 1. Glucose Effect (Catabolite Repression)

• When glucose is present, E. coli represses lac operon expression

• cAMP levels are low → CAP cannot activate transcription

• Result: β-galactosidase expression remains minimal

🔹 2. Lactose Induction

• Once glucose is depleted and lactose is added:

  • Lactose (via allolactose) binds lac repressor

  • Repressor detaches from operator

  • cAMP levels rise → CAP–cAMP activates transcription

• Result: β-galactosidase expression increases after a short lag

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Expected β-Galactosidase Expression Profile

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Correct Answer

✅ Option (D) – Profile iv

Why profile iv is correct:

• Shows negligible β-gal expression during first 12 hours

• Displays a sharp rise after 12 hours, when lactose is introduced

• Perfectly matches catabolite repression followed by lac operon induction

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Explanation of All Options

Option (i) – Incorrect

• Shows gradual β-gal expression even during glucose growth

• ❌ Contradicts catabolite repression

• Lac operon should be OFF in presence of glucose

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Option (ii) – Incorrect

• Shows high expression around 12 h followed by decline

• ❌ No biological basis for decline after lactose induction

• β-gal should remain elevated while lactose is present

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Option (iii) – Incorrect

• Shows stepwise or partial expression during glucose phase

• ❌ Ignores strong glucose-mediated repression

• β-gal should remain almost zero until lactose addition

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Option (iv) – Correct

• No expression during glucose growth

• Strong induction after lactose addition

• ✔ Accurately reflects lac operon regulation

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Final Answer

✅ Correct Option: (D) – iv