Question

RNA polymerase tends to bind to which of the following in lac operon system?

• A. Promoter region
• B. Operator region
• C. Lac I
• D. IPTG

Hint:

Always associate "Promoter" with "RNA Polymerase binding" (to start transcription) and "Operator" with "Repressor binding" (to block transcription). This distinction is a fundamental concept in operon models.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The lac operon of E. coli is a classic model of prokaryotic gene regulation.
It consists of structural genes (lacZ, lacY, lacA) and regulatory DNA sequences (promoter and operator) that control their transcription.

Step 2: Detailed Explanation:
Transcription initiation requires RNA polymerase to bind to a specific regulatory sequence on the DNA upstream of the structural genes.
Promoter Region (A): This sequence is specifically recognized by the RNA polymerase holoenzyme (via its \(\sigma\) subunit).
Once bound to the promoter, the polymerase unwinds the DNA double helix to initiate transcription. Thus, RNA polymerase binds to the promoter.
Let us review the other components:
Operator Region (B): This sequence lies between the promoter and the structural genes.
It is the binding site for the lac repressor protein.
When the repressor is bound to the operator, it physically blocks RNA polymerase from moving forward, preventing transcription.
Lac I (C): This regulatory gene encodes the lac repressor protein.
IPTG (D): This is a synthetic, non-metabolizable analog of lactose that acts as an inducer by binding to the repressor and causing it to release from the operator.

Step 3: Final Answer:
RNA polymerase binds to the promoter region in the lac operon system, corresponding to option (A).