Bacterial transcription initiation: What is the principal function of the sigma (σ) factor subunit of RNA polymerase?

Difficulty: Easy

Correct Answer: recognition of the transcriptional start sequence

Explanation:


Introduction / Context:
In bacteria, the RNA polymerase holoenzyme includes a σ factor that confers promoter specificity. Correctly assigning σ factor function is central to understanding promoter recognition and gene regulation in prokaryotes.



Given Data / Assumptions:

  • Bacterial core RNA polymerase (α2ββ′ω) requires σ for promoter binding.
  • “Start sequence” refers to promoter elements such as −35 and −10 boxes.
  • Translational signals (start/stop codons) are distinct from transcriptional signals.



Concept / Approach:
σ factors recognize and bind promoter DNA, positioning the core enzyme at the correct transcription start site (TSS). After initiation, σ often dissociates, allowing elongation. Different σ factors target distinct promoter classes, enabling adaptive gene expression.



Step-by-Step Solution:
Identify the relevant stage: initiation of transcription.Assign σ role: promoter recognition and TSS selection.Therefore, σ recognizes transcriptional start sequences.



Verification / Alternative check:
Mutations in −10/−35 elements reduce σ-dependent binding and initiation, confirming its role in start-site recognition.



Why Other Options Are Wrong:

  • Translational start/stop sequences concern ribosomes, not RNA polymerase σ.
  • Transcriptional stop sequences are recognized by rho-dependent or intrinsic terminator mechanisms, not σ.



Common Pitfalls:
Mixing translation signals with transcription signals due to similar “start/stop” wording. Keep the processes distinct.



Final Answer:
recognition of the transcriptional start sequence

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