Coordination between the molecular machineries that synthesize and decode prokaryotic mRNAs is an important layer of gene expression control known as transcription-translation coupling [1]. The leading ribosome on an mRNA can contact RNA polymerase (RNAP) to form a molecular assembly known as an ‘expressome’. This interaction contributes to the synchronization of transcription and translation elongation rates observed in E. coli. It is also important for releasing paused transcription complexes, which is required to maintain genome stability. The molecular basis of expressome assembly and its consequences for transcription and translation are poorly understood.
We determined a series of expressome structures by cryogenic electron microscopy (cryo-EM) [2]. These revealed how a coupling factor, NusG, physically bridges the transcription and translation machinery to guide nascent mRNA between the sites of synthesis and decoding. In addition, we structurally characterized an uncoupled collided state in which the ribosome releases paused RNAP.
A different, and largely unexplored, coupling mechanism is the reciprocal stimulation of translation by the transcription machinery. Using cryo-EM and single-molecule fluorescence analyses, we characterized a mechanism by which translation initiation is stimulated by RNAP. The structure of a ribosome-RNAP complex revealed RNAP and ribosomal protein S1 can cooperatively deliver the nascent mRNA for Shine-Dalgarno (SD) duplex formation. This work identifies a mechanism by which transcription-translation coupling is likely established prior to expressome formation.