Rebuilding the bridge between transcription and translation.

In Bacteria, ribosomes may bind to the nascent RNA emerging from the transcribing RNA polymerase and initiate translation. Transcription-translation coupling plays diverse roles in cellular physiology, including attenuation control, mRNA surveillance and maintenance of genome integrity. While the existence of coupling is broadly accepted, its mechanism and ubiquity are debated. Structural evidence supports mutually exclusive modes of RNA polymerase-ribosome contacts. In a model based on nuclear magnetic resonance data, NusG binds to a ribosomal protein S10 and acts as an adapter between RNA polymerase and the 30S subunit. Recent single-particle cryo electron microscopy analyses of RNA polymerase bound to 30S and 70S ribosomes revealed extensive, and very distinct, contacts which are incompatible with bridging by NusG. Saxena et al. provide the first evidence for NusG-mediated coupling in vivo. Their results demonstrate that Escherichia coli NusG interacts with the 70S ribosomes through a previously established interface and that these interactions are required for survival when translation elongation is hindered to weaken coupling. Future studies will address a likely possibility that distinct bridging mechanisms underpin context-dependent coupling in the cell.