Functional Compatibility of Elongation Factors Between Mammalian Mitochondrial and Bacterial Ribosomes: Characterization of GTPase Activity and Translation Elongation by Hybrid Ribosomes Bearing Heterologous L7/12 Proteins

The mammalian mitochondrial (mt) ribosome (mitoribosome) is a bacterial-type ribosome but has a highly protein-rich composition. Almost half of the rRNA contained in the bacterial ribosome is replaced with proteins in the mitoribosome. Escherichia coli elongation factor G (EF-G Ec) has no translocase activity on the mitoribosome but EF-G mt is functional on the E. coli ribosome. To investigate the functional equivalency of the mt and E. coli ribosomes, we prepared hybrid mt and E. coli ribosomes. The hybrid mitoribosome containing E. coli L7/12 (L7/12 Ec) instead of L7/12 mt clearly activated the GTPase of EF-G Ec and efficiently promoted its translocase activity in an in vitro translation system. Thus, the mitoribosome is functionally equivalent to the E. coli ribosome despite their distinct compositions. The mt EF-Tu-dependent translation activity of the E. coli ribosome was also clearly enhanced by replacing the C-terminal domain (CTD) of L7/12 Ec with the mt counterpart (the hybrid E. coli ribosome). This strongly indicates that the CTD of L7/12 is responsible for EF-Tu function. These results demonstrate that functional compatibility between elongation factors and the L7/12 protein in the ribosome governs its translational specificity.