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.