Nucleotide sequences of animal mitochondrial tRNAsMet possibly recognizing both AUG and AUA codons

To elucidete the role of modified nucleosides of tRNA in mitochondrial translation systems, especially with regard to their codon recognition, we purified mitochondrial tRNAsMet isolated from liver of frog, chicken and rat, and determined their nucleotide sequences. All of these tRNAsMet were found to possess 5-formylcytidine in the first letter of the anticodon, which is known to be prerequisite for bovine mt tRNAMet to decode AUA codon as well as AUG codon. These tRNA possesses two pseudeuridines in similar positions, and only chicken tRNAMet had ribothymidine at the first position of the T-loop, which is always found in the usual tRNAs. Considering that AUA codon is used as five times frequently as AUG codon in these animal mitochondrial genomes, it is deduced that 5-formylcytidine at the wobble position is essential for the recognition of both AUA and AUG codons.