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

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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.