During transcription initiation by RNA polymerase II, what event terminates the initiation phase and commits to elongation?

• A. DNA rewinds to form a closed complex.
• B. A promoter escape where RNA polymerase leaves the promoter region with nascent RNA.
• C. Initiation factors are degraded.
• D. The tau protein seals the transcription bubble.

Answer: (B)

Promoter escape marks the transition from initiation to elongation. During initiation, RNA polymerase II and the general transcription factors assemble at the promoter and unwind the DNA to form an open complex, often producing short abortive transcripts. When the polymerase synthesizes enough RNA and reorganizes its contacts with the promoter, it clears the promoter and moves downstream into the coding region. This escape commits the machine to productive elongation. The other ideas don’t capture this key transition: rewinding the DNA would shut down the open complex rather than start elongation, initiation factors aren’t typically degraded to trigger elongation, and a tau protein sealing the bubble isn’t part of this process.