Bacterial RNA polymerase – promoter specificity: Which subunit of the prokaryotic RNA polymerase holoenzyme primarily confers recognition of promoter sequences during transcription initiation?

Introduction / Context:
Bacterial RNA polymerase exists as a core enzyme that elongates RNA and a holoenzyme that includes a specificity factor. Understanding the role of the sigma (σ) factor is central to grasping how bacteria rapidly reprogram transcription in response to stress and environmental cues.

Given Data / Assumptions:

• Core enzyme (α2ββ′ω) lacks efficient promoter recognition on its own.
• Sigma factors bind core to form holoenzyme and direct it to specific promoter motifs.
• Different σ factors target different promoter consensus sequences.

Concept / Approach:
The σ subunit contacts the −10 and −35 regions (or alternative motifs) in promoter DNA, enabling closed-complex formation and promoter melting to the open complex. Swapping σ factors retargets RNAP to stress-responsive or housekeeping promoters, a key regulatory strategy in prokaryotes.

Step-by-Step Solution:
Define core vs holoenzyme. Assign promoter recognition role to σ subunit. Select the option naming sigma (σ).

Verification / Alternative check:
Genetic deletion or overexpression of specific σ factors changes promoter usage; in vitro reconstitution confirms σ-dependent promoter binding.

Why Other Options Are Wrong:
α assists assembly and activation; β/β′ form the catalytic center; ω aids assembly/stability; γ is not a standard bacterial RNAP subunit.

Common Pitfalls:
Assuming core RNAP alone targets promoters efficiently; without σ, specificity is poor.

Final Answer:
Sigma (σ) subunit.