In a particular bacterial species, temperature-sensitive conditional mutations cause expression of a wild-type phenotype at one growth temperature and a mutant phenotype at another—typically higher—temperature. Imagine that when a bacterial cell carrying such a mutation is shifted from low to high growth temperatures, RNA polymerases in the process of elongation complete transcription normally, but no new transcripts can be started. The mutation in this strain most likely affects:
a. The terminator sequence
b. The start codon
c. Sigma
d. One of the polypeptides of the core RNA polymerase

Verified step by step guidance

1. Understand the problem: The question describes a temperature-sensitive mutation in bacteria that affects transcription. At higher temperatures, RNA polymerases can complete elongation but cannot initiate new transcription. The goal is to identify which component of the transcription machinery is most likely affected by the mutation.
2. Review the transcription process: Transcription in bacteria involves several key steps: initiation, elongation, and termination. Initiation requires the RNA polymerase holoenzyme, which consists of the core RNA polymerase and a sigma factor. The sigma factor is responsible for recognizing the promoter and initiating transcription.
3. Analyze the mutation's effect: The problem states that elongation proceeds normally, meaning the core RNA polymerase is functional. However, new transcription cannot start, which suggests an issue with the initiation phase. This points to a defect in the sigma factor, as it is essential for promoter recognition and initiation.
4. Eliminate incorrect options: a) The terminator sequence is involved in ending transcription, not initiation, so it is unlikely to be affected. b) The start codon is part of translation, not transcription, so it is irrelevant here. d) The core RNA polymerase is functional during elongation, so it is not the issue.
5. Conclude the most likely answer: The sigma factor (option c) is the component most likely affected by the mutation, as it is critical for initiating transcription and recognizing promoters, which aligns with the described phenotype.

Key Concepts

RNA Polymerase Function

RNA polymerase is an enzyme responsible for synthesizing RNA from a DNA template during transcription. It binds to the promoter region of a gene to initiate transcription and elongates the RNA strand by adding nucleotides complementary to the DNA template. Understanding its role is crucial for analyzing how mutations can affect transcription initiation and elongation.

Transcription Initiation

Transcription initiation is the process where RNA polymerase binds to the promoter region of a gene, requiring specific factors such as sigma factors in bacteria. These factors help the polymerase recognize the correct start site for transcription. A mutation affecting this process would lead to the inability to start new transcripts, as described in the question.

Conditional Mutations

Conditional mutations are genetic alterations that result in a phenotype only under certain environmental conditions, such as temperature. In the context of the question, the mutation allows normal transcription at lower temperatures but disrupts it at higher temperatures, indicating that the mutation likely affects a component essential for transcription initiation or regulation at elevated temperatures.