What is the role of sigma factors in bacterial transcription initiation?

• A. Sigma factors catalyze RNA synthesis.
• B. Sigma factors recognize specific promoter sequences and recruit RNA polymerase to start transcription.
• C. Sigma factors terminate transcription.
• D. Sigma factors unwind DNA ahead of polymerase.

Answer: (B)

This question tests how bacteria start transcription and the role of sigma factors. Sigma factors provide promoter specificity to the RNA polymerase. The core enzyme by itself doesn’t efficiently find promoter sequences, so when a sigma factor binds, it forms the RNA polymerase holoenzyme that recognizes conserved promoter elements like the -35 and -10 regions. This recognition positions the enzyme at the transcription start site and helps melt the DNA to form the open complex, enabling transcription to begin. After initiation, the sigma factor typically dissociates, allowing the core enzyme to proceed with elongation.

In other words, sigma factors don’t catalyze RNA synthesis themselves, they don’t terminate transcription, and while promoter opening occurs during initiation, sigma factors are not helicases that unwind DNA on their own.