After initiation, what must RNA polymerase do to clear the promoter?

• A. Extend enough RNA to overcome promoter constraints.
• B. Initiate transcription by promoter recognition.
• C. Terminate transcription immediately.
• D. Bind to a different promoter.

Answer: (A)

The main idea here is promoter clearance during transcription initiation. After RNA polymerase and its sigma factor bind the promoter and begin RNA synthesis, the enzyme is held near the promoter by contacts with promoter DNA and the sigma factor. To switch from initiation to elongation, the growing RNA chain must become long enough to weaken these contacts and allow the polymerase to move away from the promoter. Once the RNA reaches a sufficient length—often around 8–10 nucleotides—the sigma factor typically dissociates and the enzyme enters the elongation phase, successfully clearing the promoter. This is also when abortive short transcripts are common, but eventually the polymerase escapes and proceeds with processive RNA synthesis.