Ribosome collisions in bacteria promote ribosome rescue by triggering mRNA cleavage by SmrB

Ribosome rescue pathways recycle stalled ribosomes and target problematic mRNAs and aborted proteins for degradation. In bacteria, it remains unclear how rescue pathways distinguish ribosomes stalled in the middle of a transcript from actively translating ribosomes. In a genetic screen in E. coli, we discovered a novel rescue factor that has endonuclease activity. SmrB cleaves mRNAs upstream of stalled ribosomes, allowing the ribosome rescue factor tmRNA (which acts on truncated mRNA) to rescue upstream ribosomes. SmrB is recruited by ribosome collisions. Cryo-EM structures of collided disomes from E. coli and B. subtilis reveal interactions between the 30S subunits and a possible SmrB binding site. These findings show that ribosome collisions trigger ribosome rescue in bacteria and reveal the mechanism by which this occurs. Overall design: Ribosome profiling of four strains expressing a reporter construct with a short SecM stalling motif from the plasmid "IRAGP_reporter" in four strains: Wild-type MG1655, delta-ssrA, delta-smrB, and the double knockout delta-ssrA delta-smrB.