Q.41 Determine the correctness or otherwise of the following Assertion (a) and the Reason (r).
Assertion: Isopropylthiogalactoside (IPTG) is a gratuitous inducer of lactose operon.
Reason: Gratuitous inducers are chemical analogs which behave like natural inducer but they do not serve as substrate for the enzymes that are subsequently synthesized.
Correct Answer: (A) Both (a) and (r) are true and (r) is the correct reason for (a)
IPTG induces the lac operon by binding lac repressor (LacI) with higher affinity than allolactose but resists hydrolysis by β-galactosidase, making it a gratuitous inducer. The reason defines gratuitous inducers precisely—analogs that mimic natural inducers without serving as enzyme substrates.
Option Analysis
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(A): Correct. IPTG (1-thio-β-D-galactopyranoside) binds LacI allosterically, releasing operator (O1/O2/P) without lacZYA metabolism. Definition matches exactly.
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(B): Incorrect. Reason directly explains IPTG’s gratuitous nature—non-metabolizable analog property enables sustained, titratable induction.
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(C): Incorrect. Reason true; TMG/IPTG exemplify gratuitous inducers across gram-negative bacteria.
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(D): Incorrect. Assertion true; IPTG standard in pET/BL21(DE3) systems for T7 polymerase expression.
IPTG gratuitous inducer status in lactose operon regulation defines Q.41 assertion-reason analysis, testing molecular biology fundamentals critical for recombinant protein expression in biochemical engineering.
Gratuitous Inducer Mechanism
IPTG mimics allolactose (LacI K_d ~10⁻⁶ M vs lactose ~10⁻³ M) but lacks 1-O glycosidic bond, preventing β-galactosidase hydrolysis. Sustained intracellular levels (0.1-1 mM) ensure full derepression without catabolite repression interference. TMG (methyl-β-D-galactopyranoside) shows similar behavior but weaker LacI affinity.
Lac Operon Induction Dynamics
Glucose absent + IPTG → cAMP-CAP activation + LacI release
β-gal (lacZ): lactose → allolactose + glucose
Permease (lacY): lactose/IPTG uptake
Transacetylase (lacA): detoxification
IPTG bypasses positive feedback loop, enabling precise OD₆₀₀=0.6-0.8 induction timing for maximal protein yield.
Biotechnology Applications
pET System: IPTG triggers T7 RNAP → target gene cascade (BL21(DE3)).
Titration: 0.01-1 mM optimizes soluble vs inclusion body ratios.
Autoinduction: Glucose exhaustion → lactose/IPTG phase for metabolic burden reduction.
GATE Biotechnology Context
Q.41 links Q.35-40 sequence: operon regulation (Shine-Dalgarno expression), PPI (LacI-CAP), enzyme kinetics (allosteric LacI). Master IPTG K_d, induction curves, lacY⁻ strain behavior for bioreactor optimization questions.
