Question

Escherichia coli strains (I-IV) with mutations in the lac operon were studied for $\beta$-galactosidase activity, followed by measuring its mRNA and protein levels (Graphs). The arrow indicates the time of addition of a compound (namely IPTG) that induces the lac operon. Based on the results, the correct option(s) is(are)

• A. In strain-I, the lac operon is not regulated and $\beta$-galactosidase is inactive.
• B. In strain-II, the lac operon is not regulated and $\beta$-galactosidase is inactive.
• C. In strain-IV, the lac operon is not regulated and $\beta$-galactosidase protein is not produced.
• D. In strain-III, the lac operon is regulated and $\beta$-galactosidase is active.

Answer 1

Answer: (C), (D)

C, D

Answer 2

The lac operon is an inducible operon, meaning its expression is typically low in the absence of an inducer (like IPTG) and significantly increases upon inducer addition. We need to analyze each strain based on the provided graphs for $\beta$-galactosidase mRNA, protein, and activity levels. The arrow indicates the time of IPTG addition.

1. Strain-I:

   • $\beta$-galactosidase mRNA: Levels are high and increasing from time 0, even before IPTG addition, and continue to increase after IPTG. This indicates constitutive transcription, meaning the lac operon is not regulated by IPTG.
   • $\beta$-galactosidase protein: Levels are high and increasing from time 0, similar to mRNA. This indicates constitutive protein production.
   • $\beta$-galactosidase activity: Activity is high and increasing from time 0. This indicates that the produced $\beta$-galactosidase is active.
   • Conclusion for Strain-I: The lac operon is not regulated (constitutively expressed), and $\beta$-galactosidase is active.
   • Option (A) evaluation: "In strain-I, the lac operon is not regulated and $\beta$-galactosidase is inactive." This is incorrect because $\beta$-galactosidase is active.

2. Strain-II:

   • $\beta$-galactosidase mRNA: Levels are low before IPTG addition and significantly increase after IPTG. This indicates that the lac operon is regulated and inducible.
   • $\beta$-galactosidase protein: Levels are low before IPTG addition and significantly increase after IPTG, mirroring mRNA levels. This indicates regulated protein production.
   • $\beta$-galactosidase activity: Levels are low before IPTG and remain low even after IPTG addition, despite protein production. This indicates that the produced $\beta$-galactosidase protein is inactive.
   • Conclusion for Strain-II: The lac operon is regulated, but the $\beta$-galactosidase protein is inactive.
   • Option (B) evaluation: "In strain-II, the lac operon is not regulated and $\beta$-galactosidase is inactive." This is incorrect because the lac operon is regulated.

3. Strain-III:

   • $\beta$-galactosidase mRNA: Levels are low before IPTG and significantly increase after IPTG. This indicates that the lac operon is regulated and inducible.
   • $\beta$-galactosidase protein: Levels are low before IPTG and significantly increase after IPTG, mirroring mRNA levels. This indicates regulated protein production.
   • $\beta$-galactosidase activity: Levels are low before IPTG and significantly increase after IPTG, mirroring protein levels. This indicates that the produced $\beta$-galactosidase is active.
   • Conclusion for Strain-III: The lac operon is regulated, and $\beta$-galactosidase is active. This represents a normal, inducible lac operon.
   • Option (D) evaluation: "In strain-III, the lac operon is regulated and $\beta$-galactosidase is active." This statement is correct.

4. Strain-IV:

   • $\beta$-galactosidase mRNA: Levels are high and increasing from time 0, even before IPTG addition, and continue to increase after IPTG. This indicates constitutive transcription, meaning the lac operon is not regulated by IPTG.
   • $\beta$-galactosidase protein: Levels are low before IPTG and remain low even after IPTG addition, despite high mRNA levels. This indicates that $\beta$-galactosidase protein is not produced (or produced in negligible amounts, or is highly unstable).
   • $\beta$-galactosidase activity: Levels are low before IPTG and remain low even after IPTG, consistent with no protein production.
   • Conclusion for Strain-IV: The lac operon is not regulated (constitutive transcription), but $\beta$-galactosidase protein is not produced.
   • Option (C) evaluation: "In strain-IV, the lac operon is not regulated and $\beta$-galactosidase protein is not produced." This statement is correct.

Based on the analysis, both options (C) and (D) are correct.