Structural insight into bacterial co-transcriptional translation initiation

In bacteria, transcription and translation are tightly coupled, forming a transcription-translation complex (TTC) between RNA polymerase (RNAP) and the ribosome. As nascent mRNA emerging from RNAP is susceptible to ribonuclease digestion, undesired RNA folding, and R-loop formation, immediate TTC formation is important. Here, we report the cryo-electron microscopy structures that capture the translation initiation complex assembly on transcribing RNAP. As a short mRNA emerging from RNAP, the 30S ribosomal subunit binds the RNAP on its inter-subunit side, interacting with the mRNA 5’-region and the initiator tRNA. The RNAP could relocate to the canonical mRNA-entry site of 30S, threading the mRNA into a path formed between RNAP and 30S. The subsequent 50S joining establishes a TTC. These structures illustrate the transcription-coupled translation initiation, while protecting mRNA. One-Sentence Summary Cryo-EM captures the ribosome assembly on transcribing RNA polymerase in the presence of translation initiation factors.