The role of sigma factor during bacterial transcription is to cause

Question

BlackTom AI · Accepted Answer

Sigma factor directs RNA polymerase to specific promoter sequences by recognizing and binding conserved −35 and −10 elements, so it causes the binding of RNA polymerase to the promoter.

Relevant knowledge points:
- Bacterial RNA polymerase core enzyme (α2ββ′) cannot efficiently recognize promoters on its own; association with a sigma (σ) subunit forms the holoenzyme that confers promoter specificity.
- Promoter recognition, formation of closed and open complexes, and initiation of RNA synthesis are sigma-dependent events. After initiation, sigma often dissociates allowing elongation by the core enzyme.
- Termination mechanisms (rho-dependent or rho-independent) do not involve sigma.

Step-by-step reasoning:
1. The core RNA polymerase plus sigma = RNA polymerase holoenzyme. Sigma contains domains that specifically bind conserved promoter sequences (−35 and −10 motifs).
2. Holoenzyme first forms a closed complex at the promoter (DNA double helix intact). Sigma promotes isomerization to the open complex (local DNA melting around the start site), enabling initial phosphodiester bond formation.
3. After synthesizing a short RNA (promoter clearance), sigma typically dissociates; the core enzyme continues elongation. Thus sigma’s primary role is promoter binding and initiation, not elongation or termination.

Why the correct answer is correct:
- “The binding of RNA polymerase to the promoter” directly describes sigma’s function: mediating promoter recognition and holoenzyme formation.

Why the other options are incorrect:
- “Conversion from initiation to elongation”: that transition is performed mainly by the core polymerase after promoter clearance; sigma is usually released rather than causing the conversion.
- “Rho-dependent termination”: involves the rho helicase chasing and releasing the transcript, unrelated to sigma.
- “Rho-independent termination”: depends on a GC-rich hairpin and a U-tract in the RNA, not sigma.

Therefore the correct answer is: the binding of RNA polymerase to the promoter.

The role of sigma factor during bacterial transcription is to causeSingle choice

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