In bacterial transcription, what is the primary function of the sigma (σ) factor associated with RNA polymerase? Choose the most accurate description.

Introduction / Context:
In prokaryotes, the core RNA polymerase requires an accessory factor called sigma (σ) to recognize promoters efficiently. Understanding σ-factor function clarifies how transcription start sites are selected genome-wide.

Given Data / Assumptions:

• Sigma factors transiently associate with the core polymerase.
• Promoters contain recognizable sequence motifs (for example, −35 and −10 elements in E. coli).
• Translation initiation is a separate process driven by ribosome binding (Shine–Dalgarno sequence), not by sigma factors.

Concept / Approach:
Sigma factors confer promoter specificity to the RNA polymerase holoenzyme, enabling correct binding, open complex formation, and initiation at precise sites. After initiation, σ often dissociates, and the core enzyme continues elongation.

Step-by-Step Solution:
Identify the step affected: initiation of transcription.Define σ role: promoter recognition and positioning the polymerase at the correct start site.Conclude: σ assures transcription begins at the proper point.

Verification / Alternative check:
Mutations in σ or promoter elements disrupt accurate start site usage; swapping σ factors redirects transcription to different regulons.

Why Other Options Are Wrong:
Termination involves rho or intrinsic signals, not σ. Translation start/stop are ribosome-mediated processes; σ has no role there. Proofreading is limited in RNA polymerase and is not a σ function.

Common Pitfalls:
Assuming σ remains bound during elongation or confusing translation control with transcription initiation.

Final Answer:
Assures that transcription begins at the proper point.