20. With reference to lac operon, what will be the phenotype of an E. coli strain having a genotype I^– O⁺ Z⁺ Y^– /F’ I⁺ O^C Z^– Y⁺ ?
(1) Constitutive for both β-galactosidase and lac permease
(2) Inducible for both β-galactosidase and lac permease.
(3) Inducible for β-galactosidase and constitutive for lac permease.
(4) Constitutive for β-galactosidase and inducible for lac permease.

 

The lac operon of Escherichia coli is a classic model for understanding gene regulation, especially through studies of merodiploid (partial diploid) strains. These strains, carrying two copies of the operon with different mutations, reveal how regulatory elements interact in cis and trans. Here, we analyze the phenotype of an E. coli strain with the genotype I– O+ Z+ Y– / F’ I+ OC Z– Y+, focusing on the expression of β-galactosidase (lacZ) and lactose permease (lacY).

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Genotype Breakdown

• Chromosomal copy: I– O+ Z+ Y–

  • I–: Nonfunctional repressor (cannot repress lac operon; acts in trans)

  • O+: Wild-type operator (can be repressed if repressor is present)

  • Z+: Functional β-galactosidase

  • Y–: Nonfunctional permease

• F’ (plasmid) copy: I+ OC Z– Y+

  • I+: Functional repressor (can repress both operons; acts in trans)

  • OC: Constitutive operator (cannot be repressed; acts in cis)

  • Z–: Nonfunctional β-galactosidase

  • Y+: Functional permease

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Mechanisms of Regulation

• lacI (repressor) acts in trans: A functional I+ allele on the F’ plasmid can produce repressor protein that acts on both the chromosomal and plasmid operons.

• lacO (operator) acts in cis: An OC mutation only affects the operon on which it is located, causing constitutive expression of downstream genes regardless of repressor presence.

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β-Galactosidase (lacZ) Expression

• Functional lacZ is only on the chromosome (Z+).

• Its operator is O+ (wild-type), so it can be repressed by a functional repressor.

• The F’ copy has Z– (nonfunctional), so it does not contribute to β-galactosidase activity.

Result:

• The I+ allele from the F’ plasmid provides repressor in trans, repressing the O+ Z+ chromosomal operon in the absence of inducer.

• Upon induction (e.g., with lactose), the repressor is inactivated and the O+ Z+ operon is expressed.

• β-Galactosidase expression is inducible.

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Lactose Permease (lacY) Expression

• Functional lacY is only on the F’ plasmid (Y+).

• Its operator is OC (constitutive), so it is always expressed, regardless of repressor presence.

• The chromosomal copy has Y– (nonfunctional), so it does not contribute to permease activity.

Result:

• The Y+ gene is under the control of OC, which is always ON.

• Lactose permease expression is constitutive.

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Phenotype Summary

• β-galactosidase: Inducible (expressed only in the presence of inducer)

• Lactose permease: Constitutive (always expressed)

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Correct Answer

(3) Inducible for β-galactosidase and constitutive for lac permease.

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Biological Significance

This merodiploid configuration demonstrates the distinction between cis- and trans-acting elements in gene regulation. The operator mutation (OC) acts only on its own operon, while the repressor (I+) can control both operons if the operator is wild-type. This allows for fine-tuned control and reveals the dominance relationships between regulatory mutations.

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Summary Table

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Conclusion

In the E. coli merodiploid I– O+ Z+ Y– / F’ I+ OC Z– Y+, β-galactosidase is inducible while lactose permease is constitutively expressed. This classic example illustrates the interplay of cis- and trans-acting mutations in bacterial gene regulation.

Keywords: lac operon, merodiploid, E. coli, I– O+ Z+ Y–, F’ I+ OC Z– Y+, β-galactosidase, lactose permease, inducible expression, constitutive expression, gene regulation, cis-acting, trans-acting, operon mutations.