Escherichia coli YaeJ protein mediates a novel ribosome‐rescue pathway distinct from SsrA‐ and ArfA‐mediated pathways

Summary Accumulation of stalled ribosomes at the 3′ end of mRNA without a stop codon (non‐stop mRNA) is supposed to be toxic to bacterial cells. Escherichia coli has at least two distinct systems to rescue such stalled ribosomes: SsrA‐dependent trans‐translation and ArfA‐dependent ribosome rescue. Combination of the ssrA and arfA mutations is synthetically lethal, suggesting the significance of ribosome rescue. In this study, we identified the E. coli yaeJ gene, encoding a peptide‐release factor homologue with GGQ motif, as a multicopy suppressor of the lethal phenotype of ssrA arfA double mutant. The YaeJ protein was shown to bind to ribosomes. Both in vivo and in vitro, YaeJ showed the ribosome‐rescue activity and promoted the hydrolysis of peptidyl‐tRNA residing in the stalled ribosome. Missense mutation in the GGQ motif or deletion of the C‐terminal unstructured tail abolished both the suppressor activity for ssrA arfA synthetic lethality and the ribosome‐rescue activity, suggesting the importance of these structural features. On the basis of these observations, we propose that YaeJ acts as a stop codon‐independent peptidyl‐tRNA hydrolysing factor through binding to ribosomes stalled at the 3′ end of non‐stop mRNAs. It was also suggested that ArfA and YaeJ rescue the stalled ribosomes by distinct mechanisms.