What is the primary function of the sigma subunit in bacterial RNA polymerase?

• A. It recognizes and binds to promoter regions to initiate transcription.
• B. It adds a 5' cap to the newly synthesized mRNA.
• C. It catalyzes the formation of phosphodiester bonds during RNA synthesis.
• D. It unwinds the DNA double helix ahead of the transcription bubble.

Answer: (A)

Verified step by step guidance

1. Understand that bacterial RNA polymerase is a multi-subunit enzyme responsible for synthesizing RNA from a DNA template during transcription.
2. Recognize that the core RNA polymerase enzyme consists of multiple subunits, including the beta, beta prime, alpha, and omega subunits, which together carry out the catalytic activity of RNA synthesis.
3. Identify the sigma subunit as a detachable factor that associates with the core enzyme to form the holoenzyme, which is essential for the initiation phase of transcription.
4. Know that the primary role of the sigma subunit is to recognize and bind specific DNA sequences called promoter regions, which are located upstream of the genes to be transcribed.
5. By binding to the promoter, the sigma subunit helps position the RNA polymerase correctly to start transcription at the right site, after which it typically dissociates from the core enzyme to allow elongation to proceed.