Distinct mechanisms of the human mitoribosome recycling and antibiotic resistance

Ribosomes are recycled for a new round of translation initiation by dissociation of ribosomal subunits, messenger RNA and transfer RNA from their translational post-termination complex. Here we present cryo-EM structures of the human 55S mitochondrial ribosome (mitoribosome) and the mitoribosomal large 39S subunit in complex with mitoribosome recycling factor (RRF(mt)) and a recycling-specific homolog of elongation factor G (EF-G2(mt)). These structures clarify an unusual role of a mitochondria-specific segment of RRF(mt), identify the structural distinctions that confer functional specificity to EF-G2(mt), and show that the deacylated tRNA remains with the dissociated 39S subunit, suggesting a distinct sequence of events in mitoribosome recycling. Furthermore, biochemical and structural analyses reveal that the molecular mechanism of antibiotic fusidic acid resistance for EF-G2(mt) is markedly different from that of mitochondrial elongation factor EF-G1(mt), suggesting that the two human EF-G(mt)s have evolved diversely to negate the effect of a bacterial antibiotic.