Tracking transcription-translation coupling in real-time

A central question in biology is how macromolecular machines function cooperatively. In bacteria, transcription and translation occur in the same cellular compartment and can be physically and functionally coupled. While several recently published high-resolution structures of the ribosome-RNA polymerase (RNAP) complex provided first mechanistic insight into the coupling process, we do not know how these structural snapshots are placed along a dynamic reaction trajectory. Here, we reconstitute a complete active transcription-translation system and develop multi-color single-molecule fluorescence microscopy experiments to directly and simultaneously track transcription elongation, translation elongation and the physical and functional coupling between the ribosome and the RNAP in real-time. Our data show that the ribosome slows down while colliding with the RNAP and that coupling following a collision becomes less efficient. Unexpectedly, physical coupling can occur with hundreds of nucleotides of intervening mRNA between both machineries by mRNA looping, and increases in efficiency in presence of NusG. We detect active transcription elongation during mRNA looping and show that NusA-paused RNAPs can be activated by the ribosome by long-range physical coupling. We provide an alternative explanation on how the ribosome can rescue RNAP from frequent pausing without requiring collisions by a closely trailing ribosome. Our data mechanistically highlight an example of how macromolecular machines central to gene expression physically and functionally cooperate. ### Competing Interest Statement The authors have declared no competing interest.