Alternative sigma factors and timing: A specific ________ factor enables RNA polymerase to bind late promoters and transcribe late genes (for example, during certain phage infections or stress responses).

Introduction / Context:
Bacteria and bacteriophages use alternative sigma factors to redirect RNA polymerase to distinct promoter sets. This allows temporal control of gene expression, such as early versus late genes in phage development or stress-responsive regulons in the bacterial host.

Given Data / Assumptions:

• RNA polymerase core enzyme pairs with different sigma subunits.
• Late promoters have sequence motifs recognized by a specific sigma factor.
• Changing sigma changes promoter specificity without altering the core catalytic machinery.

Concept / Approach:
Sigma factors act as interchangeable specificity modules. During phage infection, a phage-encoded sigma or a host sigma modified by the phage can target late promoters that control capsid assembly genes, lysis genes, and other late functions. In bacteria, alternative sigmas (for heat shock, stationary phase, flagella, etc.) similarly reprogram transcriptional priorities.

Step-by-Step Solution:
Recognize that promoter specificity is governed by sigma, not beta, delta, or gamma subunits. Recall classic examples where late gene transcription requires a different sigma factor. Select “Sigma.”

Verification / Alternative check:
Biochemical reconstitution shows that swapping sigma subunits alters which promoter sequences are efficiently recognized in vitro and in vivo.

Why Other Options Are Wrong:

• Beta: A core catalytic subunit; does not confer alternate promoter specificity by itself.
• Delta / Gamma / Theta: Not the standard promoter-recognition factors in bacterial RNA polymerase.

Common Pitfalls:
Confusing transcription factor proteins that bind DNA with sigma factors; sigma is a component of the holoenzyme, not a separate DNA-binding regulator acting alone.

Final Answer:
Sigma.