Kinetic control of mammalian transcription elongation

Transcription elongation by RNA polymerase II is an integral step in eukaryotic gene expression. The speed of Pol II is controlled by a multitude of elongation factors, but the regulatory mechanisms remain incompletely understood, especially for higher eukaryotes. In this work, we developed a single-molecule platform to visualize the dynamics of individual mammalian transcription elongation complexes (ECs) reconstituted from purified proteins. This platform enabled us to follow the elongation and pausing behavior of EC in real time and dissect the role of each elongation factor in the kinetic control of Pol II. We found that the mammalian EC harbors multiple gears dictated by its associated factors and phosphorylation status. Moreover, the elongation factors are not functionally redundant but act hierarchically and synergistically to achieve optimal elongation activity. Such exquisite kinetic regulation may underline the major speed-changing events during the transcription cycle and enable cells to adapt to a changing environment.