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. Understand the role of each component listed in the options in the process of transcription in bacteria. The terminator sequence is involved in ending transcription, the start codon is part of mRNA and involved in translation initiation, sigma factors are involved in the initiation of transcription by aiding the binding of RNA polymerase to promoters, and the core RNA polymerase is responsible for the elongation phase of transcription.
2. Analyze the effect of the mutation described: it allows RNA polymerase to complete transcription of already started transcripts upon a temperature shift, but prevents the initiation of new transcripts.
3. Consider which component's malfunction could prevent the initiation of new transcripts while allowing ongoing transcription to complete. This would suggest a defect in a component involved specifically in the initiation phase of transcription.
4. Recall that sigma factors in bacteria are responsible for the recognition of promoter sequences and are crucial for the initiation of transcription. They are not part of the core RNA polymerase that carries out the elongation of the RNA chain.
5. Conclude that the mutation likely affects a component that is critical to the initiation of transcription but not required for the elongation phase. Given the options, the sigma factor is the most plausible candidate affected by the mutation.

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 how RNA polymerase operates, including its ability to initiate and elongate RNA transcripts, is crucial for analyzing mutations that affect transcriptional processes.

Sigma Factor Role

The sigma factor is a protein that associates with RNA polymerase to facilitate the initiation of transcription in bacteria. It helps the RNA polymerase recognize and bind to specific promoter sequences on the DNA, allowing for the correct initiation of transcription. A mutation affecting the sigma factor could impair the ability of RNA polymerase to start new transcripts, particularly under different temperature conditions.

Conditional Mutations

Conditional mutations are genetic alterations that result in a phenotype that is only expressed under certain environmental conditions, such as temperature. In the context of the question, a temperature-sensitive mutation allows the organism to exhibit a wild-type phenotype at lower temperatures while displaying a mutant phenotype at higher temperatures. This concept is essential for understanding how environmental factors can influence gene expression and the functional consequences of specific mutations.