Concurrent fast and slow cycling of a transcriptional activator at an endogenous promoter.

For gene regulation, some transcriptional activators bind periodically to promoters with either a fast (approximately 1 minute) or a slow (approximately 15 to 90 minutes) cycle. It is uncertain whether the fast cycle occurs on natural promoters, and the function of either cycle in transcription remains unclear. We report that fast and slow cycling can occur simultaneously on an endogenous yeast promoter and that slow cycling in this system reflects an oscillation in the fraction of accessible promoters rather than the recruitment and release of stably bound transcriptional activators. This observation, combined with single-cell measurements of messenger RNA (mRNA) production, argues that fast cycling initiates transcription and that slow cycling regulates the quantity of mRNA produced. These findings counter the prevailing view that slow cycling initiates transcription.