Required for rescue of stalled ribosomes mediated by trans-translation. Binds to transfer-messenger RNA (tmRNA), required for stable association of tmRNA with ribosomes (Probable). tmRNA and SmpB together mimic tRNA shape, replacing the anticodon stem-loop with SmpB. tmRNA is encoded by the ssrA gene; the 2 termini fold to resemble tRNA(Ala) and it encodes a 'tag peptide', a short internal open reading frame. During trans-translation Ala-aminoacylated tmRNA acts like a tRNA, entering the A-site of stalled ribosomes, displacing the stalled mRNA. The ribosome then switches to translate the ORF on the tmRNA; the nascent peptide is terminated with the 'tag peptide' encoded by the tmRNA and targeted for degradation. The ribosome is freed to recommence translation, which seems to be the essential function of trans-translation (PubMed:19043582). The tmRNA-SmpB complex associates with stalled 70S ribosomes. By growth at pH 5.0 in exponential phase. Essential, it cannot be deleted. Depletion experiments show loss of growth after 12 hours, cells remain viable for a further 12 hours before they can no longer be rescued by induction of smpB. Similarly, the gene for tmRNA (ssrA) cannot be deleted; mutations in the tmRNA show that it is the ribosome rescue function and not protein tagging and subsequent degradation that is essential for viability. Mutant tmRNAs that affect protein tagging are however more susceptible to antibiotic stress and less naturally competent for DNA transformation. Belongs to the SmpB family.