Ribosomal protein bL27 protects translating ribosomes from tmRNA-SmpB

Bacterial ribosomal protein bL27 is universally conserved, and its amino terminus is adjacent to the peptidyl transfer center, yet its roles in translation remain unclear. Combining genetics, biochemistry, and molecular dynamics, we show that bL27 has an unexpected role in preventing transfer-messenger RNA (tmRNA)-small protein B (SmpB), molecules involved in the trans-translation bacterial ribosome rescue mechanism, from interfering with protein synthesis. Deletion of the bL27 gene causes a 10,000-fold decrease in viability, and this defect is partially rescued by deletion of the gene encoding tmRNA. Addition of tmRNA-SmpB to in vitro translation reactions decreases the rate of protein synthesis by ribosomes lacking bL27 but has no effect on wild-type ribosomes. Molecular dynamics simulations also indicate that bL27 can slow the movement of tmRNA on the ribosome. These data link trans-translation and bL27 and support a model in which the amino terminus of bL27 acts as a gatekeeper to prevent tmRNA from sterically interfering with tRNA (transfer RNA) on the ribosome.