The ability of bovine mitochondrial transfer RNA^Met to decode AUG and AUA codons

The ability of bovine mitochondrial tRNA^Met with the anticodon f⁵CAU (where f⁵C is 5-formylcytidine) to decode AUG and AUA codons was examined in a codon-dependent ribosomal binding assay. The AUG codon stimulated the binding of Met-tRNA^Met to mitochondrial ribosomes in the presence of EF-Tu/Ts_mt. In contrast, the AUA codon did not promote the binding to mitochondrial Met-tRNA to the ribosome. To investigate the translation of the AUG and AUA codons more fully, an in vitro translation system from bovine liver mitochondria was developed. The activity of this system was greatly enhanced by the addition of 1 mM spermine and reached about half the activity observed with a comparable translational system from E. coli. Two types of mRNA containing either AUG or AUA codons were synthesized using T7 RNA polymerase to trans cribe their chemically synthesized genes. In the E coli system, the AUG-containing mRNA was translated as Met and the AUA-containing mRNA was translated as Ile. The AUG-containing mRNA but not the AUA-containing mRNA was translated as Met by the mitochondrial translational system. The process by which the AUA codon is translated as Met in the mitochondrial system remains to be clarified.