A previously uncharacterized role for small protein B (SmpB) in transfer messenger RNA-mediated trans-translation

SsrA is a versatile RNA molecule found in all bacteria that functions as both a tRNA and an mRNA. SsrA rescues ribosomes stalled on damaged mRNAs and directs the tagging and degradation of their aberrant protein products. Small protein B (SmpB) is required for all known activities of SsrA. The two known functions of SmpB are binding SsrA RNA and promoting stable association of the SmpB-SsrA complex with 70S ribosomes. Using mutational analysis and biochemical experiments, we have discovered a previously uncharacterized SmpB function. This function is required for a step in the tagging process downstream of SsrA binding and ribosome association but before transpeptidation of the SsrA-linked alanine and establishment of the SsrA reading frame. Our results clearly demonstrate that residues in the C-terminal tail of SmpB confer a hitherto unrevealed function that is essential for trans-translation. Based on these results, we propose that upon binding stalled ribosomes, the unstructured C-terminal tail of SmpB acquires contacts that are critical for productive accommodation of SsrA into the ribosomal A site.