Which sequence describes the initiation of transcription in bacteria?

• A. RNA polymerase holoenzyme binds promoter; first NTPs are added; DNA unwinds to form open complex; sigma factor released; promoter clearance.
• B. DNA unwinds to form open complex; RNA polymerase holoenzyme binds promoter; first NTPs are added; sigma factor released; promoter clearance.
• C. RNA polymerase holoenzyme binds promoter; DNA unwinds to form open complex; first NTPs are added; sigma factor released; promoter clearance.
• D. Sigma factor released before promoter binding; promoter clearance.

Answer: (C)

Initiation in bacteria starts when the RNA polymerase holoenzyme—the core enzyme plus the sigma factor—binds to the promoter, using the sigma factor to recognize promoter elements. This binding forms the closed complex and correctly positions the enzyme at the start site. Next, the DNA around the promoter unwinds to form the open complex, allowing the template strand to be exposed for transcription to begin. Once the open complex is established, the first nucleoside triphosphates are incorporated to start RNA synthesis. As the nascent RNA grows to a short length, the sigma factor is released from the core enzyme, enabling promoter clearance and the transition into productive elongation. This sequence—binding to the promoter, opening the DNA, initiating with NTPs, releasing the sigma factor, and then promoter clearance—matches how initiation proceeds. Alternatives that suggest unwinding occurs before binding, or that sigma factor release happens before promoter binding, don’t fit the actual order of events.