Role of sigma factor in bacterial transcription: Which statement correctly describes the primary function of the sigma (σ) factor in RNA polymerase holoenzyme?

Introduction / Context:
Bacterial RNA polymerase consists of a core enzyme and a sigma factor that together form the holoenzyme. Sigma factors are specificity subunits that determine which promoters are efficiently recognized and transcribed under given conditions.

Given Data / Assumptions:

• Sigma factor is required for efficient promoter recognition and transcription initiation.
• Elongation is carried out primarily by the core enzyme after sigma dissociates.
• Termination involves separate sequence features or factors (e.g., rho protein).

Concept / Approach:
The sigma factor binds characteristic promoter elements (−35 and −10 regions). By lowering the affinity of RNA polymerase for non-promoter DNA and increasing its affinity for promoter DNA, sigma factors ensure accurate start-site selection. Different sigma factors redirect RNA polymerase to distinct promoter subsets in response to stress or developmental cues.

Step-by-Step Solution:
Define sigma's role as promoter-targeting specificity. Exclude catalytic elongation functions and termination recognition, which are separate. Choose the statement describing promoter recognition and initiation.

Verification / Alternative check:
In vitro, core RNA polymerase without sigma binds DNA nonspecifically and initiates poorly; addition of sigma restores correct promoter-dependent initiation.

Why Other Options Are Wrong:

• Termination recognition: Not sigma's principal role.
• Pyrophosphate cleavage: Chemistry of phosphodiester formation is intrinsic to core enzyme, not sigma.
• DNA restriction / Helicase: Different enzymes entirely.

Common Pitfalls:
Assuming sigma stays bound throughout elongation; it most often dissociates after initiation, allowing promoter escape.

Final Answer:
Recognizes and binds promoter elements to direct initiation.