Q49.Given below are two statements:

Statement I: In prokaryotes all types of RNAs are transcribed by the holoenzyme, that consists of several polypeptides (Two α, One β, & One β′ and one σ).

Statement II: The start signals on DNA are called promoters and represent the initial binding site for the RNA polymerase.

In the light of the above statements, choose the correct answer from the options given below:

(A) Both Statement I and Statement II are correct
(B) Both Statement I and Statement II are incorrect
(C) Statement I is correct but Statement II is incorrect
(D) Statement I is incorrect but Statement II is correct

The correct answer is option (D): Statement I is incorrect but Statement II is correct. Statement I inaccurately describes prokaryotic transcription machinery, while Statement II correctly identifies promoters as RNA polymerase binding sites.

Statement Analysis

Statement I (Incorrect): Prokaryotic RNA polymerase core enzyme (2α, β, β’, ω) transcribes all RNAs. The holoenzyme (core + σ factor) handles only promoters with σ recognition (rRNA, tRNA, some protein-coding). Housekeeping σ70 transcribes ~80% genes, but alternative σ factors (σ32 heat shock, σ54 nitrogen) or core alone handle specialized/non-promoter transcription.

Statement II (Correct): Promoters contain -10 (TATAAT) and -35 (TTGACA) boxes as initial binding sites for RNA polymerase holoenzyme’s σ subunit, enabling specific transcription initiation.

Option Breakdown

• (A) Both correct: Wrong. Statement I overgeneralizes holoenzyme use.

• (B) Both incorrect: Wrong. Statement II precisely describes promoters.

• (C) I correct, II incorrect: Wrong. Statement I fails; core enzyme transcribes without σ.

• (D) I incorrect, II correct: Correct. Holoenzyme ≠ universal; promoters = binding sites.

Prokaryotic RNA polymerase holoenzyme does NOT transcribe all RNAs—only promoters serve as initial binding sites for RNAP. Q49 clarifies core enzyme (2αββ’ω) handles all transcription, holoenzyme (core+σ) promoter-specific.

Critical Transcription Distinction

Core enzyme (2α, β, β’, ω ~400 kDa) synthesizes all prokaryotic RNAs:

• rRNA, tRNA (σ-independent)

• Protein-coding (various σ factors)

• Some antisense RNAs

Holoenzyme (core + σ) binds promoters (-10/-35 boxes) for specific initiation. σ70 transcribes housekeeping; σ32 heat shock; σ54 nitrogen fixation.

Why Statement I Fails

“All types of RNAs are transcribed by the holoenzyme” ignores:

• rRNA/tRNA synthesis by core alone

• Alternative σ factors for stress genes

• σ release post-initiation (core elongates)

Exam gold: Core = universal transcriber; holoenzyme = promoter specialist. σ dissociates after ~10 nt.