Kinetic decoupling of early transcription reveals Mediator regulates paused RNA polymerase II stability

The Mediator complex serves as a crucial coactivator for RNA polymerase II (RNAPII) transcription initiation, yet its role in regulating paused RNA polymerase II and Positive transcription elongation factor b (P-TEFb) recruitment has been unclear. Researchers have struggled to study these dynamic processes due to secondary effects from prolonged protein depletion and limitations in steady-state kinetic models, which combine paused RNAPII turnover with initiation rates.

To separate Mediator's post-initiation functions from its role in assembling the pre-initiation complex, researchers created an auxin-inducible degron system in Drosophila cells for rapid, subunit-specific depletion of the Mediator complex. By combining nascent transcriptomics (SLAM-seq) with time-resolved, high-resolution mapping of RNAPII (ChIP-exo) following initiation inhibition, they determined paused polymerase kinetics independently of RNAPII influx.

Acute depletion of the structural MED14 or the metazoan-specific MED26 subunits led to decreased RNAPII recruitment and nascent transcription. However, despite substantial loss of promoter-proximal RNAPII, P-TEFb remained associated with chromatin, often increasing. This suggests Mediator and RNAPII are not globally required to recruit and maintain P-TEFb occupancy at this stage.

Moreover, while steady-state occupancy metrics indicated a faster pause release in the absence of Mediator, kinetic modeling revealed an increased paused RNAPII half-life. These findings demonstrate that steady-state metrics can misrepresent pausing dynamics due to initiation influence, and that Mediator does not act as a universal recruitment factor for P-TEFb.

Instead, Mediator regulates paused RNAPII turnover to ensure efficient early transcription kinetics.