9. Mutation in which subunit of RNA polymerase holoenzyme would have defect in correct initiation of transcription?
(1) α subunit                 (2) β subunit
(3) β’ subunit                (4) σ-subunit

Mutation in which subunit of RNA polymerase holoenzyme would result in a defect in the correct initiation of transcription?

1. α subunit

2. β subunit

3. β′ subunit

4. σ-subunit

Correct Answer: (4) σ-subunit

Explanation:

Structure of Prokaryotic RNA Polymerase Holoenzyme:

• Core enzyme: α₂ββ′ω

• Holoenzyme = Core enzyme + σ-subunit

Role of σ-subunit (Sigma Factor):

• The σ-subunit is crucial for the initiation of transcription.

• It guides the core RNA polymerase to the specific promoter sequences (−35 and −10 regions).

• Without the σ-subunit, the polymerase cannot recognize the promoter → initiation fails.

• Once transcription starts, the σ-factor usually dissociates from the complex.

Why Other Options Are Incorrect:

Conclusion:

• The σ-subunit ensures that transcription begins at the right site by recognizing the promoter.

• Mutation in this subunit → transcription initiation failure.

CSIR NET Takeaway:

• Know the roles of RNA polymerase subunits:

  • σ-subunit = promoter recognition

  • β & β′ = catalytic core

  • α = structural and regulatory support

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