Bridging the gap: When transcription meets translation

Recruitment of the small ribosomal subunit (30S) to messenger RNA (mRNA) marks a key step in bacterial translation initiation. Recruitment begins with a 30S standby site binding single-stranded mRNA regions at or near the translation initiation region (TIR). Subsequently, the mRNA is accommodated into the 30S mRNA binding channel to initiate translation. An essential part of accommodation is the recognition and proper positioning of the start codon to establish the correct reading frame. This is often facilitated by the Shine-Dalgarno (SD) sequence. Recent structural and biochemical studies provided snapshots of the first steps of 30S recruitment to a nascent mRNA preceding accommodation, which likely reflect similar events for fully transcribed mRNAs. These studies suggest that a transcribing RNA polymerase (RNAP) promotes 30S recruitment to the nascent mRNA via two distinct pathways. In one, RNAP cooperates with ribosomal protein bS1, which captures the mRNA and channels it toward the anti-Shine-Dalgarno (aSD) sequence for base pairing. In the other, direct coupling between RNAP and the 30S, mediated by transcription factor NusG, facilitates an alternative mRNA delivery pathway. We explore the current understanding of mRNA delivery, highlighting different modes of 30S recruitment to the nascent mRNA, the role of bS1, and how this leads to the establishment of transcription-translation coupling.