Supramolecular complexes involved in the coordination of transcription and translation.

Single particle cryo-electron microscopy (cryo-EM) allows us to obtain structural information of increasingly complex and dynamic assemblies. The molecular machines, which carry out gene expression, have been studied in isolation but are often organized in supramolecular complexes to coordinate their functions and regulate each other. Furthermore, new functions emerge as a consequence of this organization, which cannot be easily predicted from structures of the individual machineries. A prominent example is the translation of prokaryotic messenger RNAs by the ribosome, which initiates while they are still being transcribed by RNA polymerase (RNAP). The pioneering ribosome can approach RNAP, forming a supramolecular complex known as the expressome. We previously obtained a series of cryo-EM structures representing decoupled, coupled and collided expressome states, which allowed us to propose roles for coupling of RNAP and the ribosome and provided a framework for studies on the coordination of transcription and translation in bacteria. However, mutual coordination is not restricted to the elongation phase of translation because RNAP can also stimulate translation initiation. We used in vitro single-molecule fluorescence methods and cryo-EM to investigate how translation initiation can be promoted by RNAP. RNAP accelerates binding of the 30S ribosomal subunit to mRNA and this requires ribosomal protein S1 (bS1). The structure of a ribosome-RNAP complex revealed RNAP and bS1 can cooperatively deliver the nascent mRNA for Shine-Dalgarno (SD) duplex formation. Additional structural models showed a reorientation of the SD duplex accompanies the transition from the docking of an mRNA to its accommodation in the decoding center. Thus, transcription-translation coupling can initiate by direct contact between gene expression complexes through bS1.