bL9 prevents recoding in collided ribosomes

When a ribosome stalls and an upstream ribosome collides with it, the resulting interface recruits factors that initiate ribosome rescue pathways. Recent cryo-EM structures of collided ribosomes from bacteria reveal that bL9 from the leading ribosome reaches back to contact the 30S subunit of the trailing ribosome. The role of bL9 in translation is poorly understood; while bL9 mutants show high levels of recoding (i.e. frameshifting and hopping), the mechanism has remained unclear. Here we show that bL9 functions in the context of ribosome collisions: loss of bL9 destabilizes the interaction between collided ribosomes and sensitizes cells to antibiotics that induce collisions such as erythromycin and fusidic acid. Moreover, luciferase reporters reveal that frameshifting and hopping levels increase in strains lacking bL9 (or with point mutants in bL9 at the collision interface) but only in the context of ribosome collisions. These findings demonstrate that in wild-type bacterial ribosomes, bL9 prevents frameshifting, hopping, and other deleterious recoding events following collisions.