Phage SPO1 sigma cascade: If a frameshift mutation disrupts the sigma subunit gene that is transcribed as part of the SPO1 early gene set, what is the immediate consequence for the transcription program?

Introduction / Context: Bacillus phage SPO1 uses a sigma-factor cascade: the host RNA polymerase first transcribes phage early genes, including a phage-encoded sigma that reprograms polymerase to transcribe middle genes, followed by factors for late gene transcription.

Given Data / Assumptions:

• The sigma subunit required for middle gene transcription is produced from early gene expression.
• A frameshift mutation in this sigma gene will yield a nonfunctional sigma protein.
• Without the functional sigma, the transition from early to middle transcription fails.

Concept / Approach: Sigma factors confer promoter specificity. If the middle-phase sigma is defective, RNA polymerase cannot recognize middle promoters, blocking transcription of middle genes and preventing the normal cascade.

Step-by-Step Solution: Establish dependency: middle transcription requires the early-produced sigma. Apply mutation effect: frameshift → loss of function → failure to bind middle promoters. Conclude: middle genes are not transcribed.

Verification / Alternative check: Genetic studies of SPO1 and related phages show that disrupting phase-specific sigmas halts progression to the next transcription class, validating the cascade model.

Why Other Options Are Wrong: Translation of early genes is unaffected initially; immediate lysis is not a direct consequence; full compensation by other sigmas does not generally occur; late gene enhancement without middle transcription contradicts the cascade order.

Common Pitfalls: Confusing sigma factor function (promoter recognition) with general transcription factors; assuming redundancy where specificity is required.

Final Answer: Middle genes will not be transcribed because the required sigma factor is nonfunctional.