Required for rescue of stalled ribosomes mediated by trans-translation (By similarity). Binds to and required for stability of mature tmRNA, which it protects from degradation by RNase R (rnr) (PubMed:12511504). Required for correct localization of tmRNA (PubMed:19805312). In this organism the ssrA gene for tmRNA is circularly permutated and is processed into 2 pieces to function (PubMed:10884408). Binds to transfer-messenger RNA (tmRNA), required for stable association of tmRNA with ribosomes. 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. The tmRNA-SmpB complex associates with stalled 70S ribosomes (By similarity). Colocalizes with tmRNA in regularly spaced foci indicative of a helix-like structure: in the absence of tmRNA this protein still localizes correctly. Although RNase R (rnr) localizes in a similar helix-like pattern, the 2 helices are out of phase. Fluctuates during the cell cycle, being low in early swarmer cells and rising dramatically during the G1-S transition (when tmRNA accumulates). Decreases during DNA replication in stalked cells to rise again as cell division starts, reflecting the stability of tmRNA at all stages of the cell cycle (at protein level). Delays the cell cycle during the G1-to-S transition (same phenotype as an ssrA deletion), a 10-fold decrease in levels of mature tmRNA (PubMed:12511504). tmRNA is incorrectly localized in cells missing this gene (PubMed:19805312). Belongs to the SmpB family.