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 sequenceb. the start codonc. sigmad. one of the polypeptides of the core RNA polymerase

Verified step by step guidance

1. Identify the role of each option in the transcription process: the terminator sequence, the start codon, sigma factor, and core RNA polymerase polypeptides.
2. Understand that the terminator sequence is involved in ending transcription, not starting it, so it is unlikely to be affected if elongation completes normally.
3. Recognize that the start codon is part of the mRNA and is involved in translation initiation, not transcription initiation, so it is not relevant to the problem.
4. Recall that the sigma factor is responsible for the initiation of transcription by helping RNA polymerase bind to the promoter, which is crucial for starting new transcripts.
5. Consider that the core RNA polymerase polypeptides are involved in the elongation phase of transcription, which is functioning normally, so they are less likely to be the issue.

Key Concepts

Temperature-sensitive mutations

Temperature-sensitive mutations are genetic alterations that result in a phenotype that is dependent on the environmental temperature. In this context, the mutation allows the organism to express a wild-type phenotype at lower temperatures while exhibiting a mutant phenotype at higher temperatures, indicating that the mutation affects a temperature-sensitive process, such as protein function or stability.

RNA polymerase function

RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template during transcription. It has distinct phases, including initiation, elongation, and termination. In the scenario described, the ability of RNA polymerases to complete elongation but not initiate new transcripts suggests that the mutation may impact the initiation phase, possibly by affecting the sigma factor or other components necessary for starting transcription.

Sigma factor

The sigma factor is a protein that associates with RNA polymerase to facilitate the initiation of transcription by recognizing and binding to specific promoter sequences on the DNA. If the mutation affects the sigma factor, it could impair the ability of RNA polymerase to initiate new transcription at higher temperatures, leading to the observed inability to start new transcripts while allowing ongoing elongation of existing ones.