A translational riboswitch coordinates nascent transcription-translation coupling.

Bacterial messenger RNA (mRNA) synthesis by RNA polymerase (RNAP) and first-round translation by the ribosome are often coupled to regulate gene expression, yet how coupling is established and maintained is ill understood. Here, we develop biochemical and single-molecule fluorescence approaches to probe the dynamics of RNAP-ribosome interactions on an mRNA with a translational preQ1-sensing riboswitch in its 5' untranslated region. Binding of preQ1 leads to the occlusion of the ribosome binding site (RBS), inhibiting translation initiation. We demonstrate that RNAP poised within the mRNA leader region promotes ribosomal 30S subunit binding, antagonizing preQ1-induced RBS occlusion, and that the RNAP-30S bridging transcription factors NusG and RfaH distinctly enhance 30S recruitment and retention, respectively. We further find that, while 30S-mRNA interaction significantly impedes RNAP in the absence of translation, an actively translating ribosome promotes productive transcription. A model emerges wherein mRNA structure and transcription factors coordinate to dynamically modulate the efficiency of transcription-translation coupling.