Armless hairpin-like tRNAs in Romanomermis culicivorax: Evolutionary adaptation of a mitochondrial elongation factor EF-Tu

tRNAs are central players in translation, delivering cognate amino acids to the ribosome. To fulfill this essential function, secondary and tertiary structures of tRNAs are highly conserved. In metazoan, however, several mitochondrial tRNAs show strong structural deviations and lack D- or T-arms. As these elements are important for the interaction with tRNA-binding proteins, these proteins are adapted to recognize such unusual targets. A prominent example is mitochondrial EF-Tu, delivering aminoacylated tRNAs to the ribosome. In nematode mitochondria, two variants of mt-EF-Tu exist. While mt-EF-Tu2 is specific for D-armless mt-tRNA^Ser, mt-EF-Tu1 recognizes the remaining 20 tRNAs. The most bizarre mt-tRNAs are found in Romanomermis culicivorax, where hairpin-like structures were described lacking both D- and T-arm. To ensure functional translation with such extremely reduced tRNAs, the corresponding mt-EF-Tu1 must have undergone a further adaptation. In a comparative analysis, the tRNA-binding behavior of recombinant mitochondrial EF-Tu1 versions from several nematodes was investigated. They all carry a C-terminal extension that is required for an efficient interaction with non-canonical tRNAs. Furthermore, in mt-EF-Tu1 from R. culicivorax and Caenorhabditis elegans, a basic residue in domain III was identified that represents an additional adaptation in the transition from canonical towards hairpin-like tRNA substrates. The results indicate that nematode mt-EF-Tu1 proteins are in principle able to interact with hairpin-like tRNAs, although such transcripts are only found in some of these species. Hence, concerning mt-EF-Tu, the evolutionary stage is set for a further truncation of mitochondrial tRNAs towards armless structures.