Question

Which one of the following is a inhibitor genes in lac operon?

• A. Gene $i$
• B. Gene $z$
• C. Gene $y$
• D. Gene $a$

Hint:

Common misconception:
$\bullet$ In lac operon, '$i$' stands for inhibitor (NOT inducer).
$\bullet$ Structural genes: $z$ ($\beta$-galactosidase), $y$ (permease), $a$ (transacetylase).

Answer 1

The Correct Option is A

Concept:
The lac operon in Escherichia coli, elucidated by François Jacob and Jacques Monod, is a polycistronic transcriptional unit controlled by a shared promoter and regulatory elements.

Step 1: Genetic Organization of the lac Operon:
The lac operon comprises a regulatory gene along with three adjacent structural genes:
1. Regulatory Gene ($i$ gene): The letter '$i$' is derived from the word inhibitor. It codes constitutively for the lac repressor protein.
2. Structural Gene $z$: Transcribes and codes for the enzyme $\beta$-galactosidase, which hydrolyzes lactose into glucose and galactose.
3. Structural Gene $y$: Codes for $\beta$-galactoside permease, which increases membrane permeability to lactose.
4. Structural Gene $a$: Codes for $\beta$-galactoside transacetylase.

Step 2: Mechanism of Action of the Inhibitor Gene:
The product of the $i$ gene (the active repressor monomer that forms a tetramer) binds to the operator ($O$) region in the absence of an inducer (such as lactose or allolactose).
Binding to the operator physically impedes RNA polymerase from initiating transcription of the downstream structural genes $z, y,$ and $a$.
Because it inhibits transcription under uninduced conditions, gene $i$ is called the inhibitor gene.

Step 3: Final Answer Formulation:
Therefore, the inhibitor gene in the lac operon is Gene $i$.