Molecular basis of mRNA delivery to the bacterial ribosome

Protein synthesis begins with the formation of a ribosome-messenger RNA (mRNA) complex. In bacteria, the small ribosomal subunit (30 S ) is recruited to many mRNAs through base pairing with the Shine-Dalgarno (SD) sequence and RNA binding by ribosomal protein bS1. Translation can initiate on nascent mRNAs, and RNA polymerase (RNAP) can promote the recruitment of the pioneering 30 S . Here, we examined 30 S recruitment to nascent mRNAs using cryo–electron microscopy, single-molecule fluorescence colocalization, and in-cell cross-linking mass spectrometry. We show that bS1 delivers the mRNA to the ribosome for SD duplex formation and 30 S activation. Additionally, bS1 and RNAP stimulate translation initiation. Our work provides a mechanistic framework for how the SD duplex, ribosomal proteins, and RNAP cooperate in 30 S recruitment to mRNAs and establish transcription-translation coupling.