When does RNA polymerase II typically leave the promoter region during transcription?

RNA polymerase II typically leaves the promoter region during transcription after the transcript reaches a length of approximately 30 nucleotides. This marks a critical transition from the initiation phase of transcription to the elongation phase.

During the initiation stage, RNA polymerase II binds to the promoter region, and it is involved in synthesizing a short RNA chain. Once the RNA transcript is long enough, around 30 nucleotides, the enzyme undergoes a conformational change that allows it to disengage from the promoter. This transition is essential for effective elongation and progression through the rest of the transcription unit.

The correct timing for RNA polymerase II to leave ensures that proper processing of the mRNA can occur as it moves away from the promoter site, allowing the polymerase to proceed with elongating the transcript and reducing the chances of re-binding prematurely to the promoter. This facilitates efficient gene expression regulation and the production of a complete RNA molecule.