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

• A. Recognizes promoter sequences at -35 and -10
• B. Catalyzes RNA synthesis
• C. Unwinds DNA and stabilizes open complex
• D. Terminates transcription

Answer: (A)

Promoter recognition by sigma factor is what sets transcription in motion for bacteria. The sigma factor binds to the RNA polymerase core enzyme to form the holoenzyme, and this complex specifically recognizes promoter sequences located about 35 and 10 bases upstream of the start site. Those contacts with the -35 and -10 elements position the enzyme correctly and help unwind the DNA to create the open complex where transcription begins. After initiation, the sigma factor is usually released so the core RNA polymerase can proceed with elongation. The other functions described—catalyzing RNA synthesis, unwinding the DNA and stabilizing the open complex as the primary role of sigma, or terminating transcription—belong to other parts of the transcription machinery or to different steps, not to the main role of the sigma factor.