Convergent evolution of AUA decoding in bacteria and archaea.

Deciphering AUA codons is a difficult task for organisms, because AUA and AUG specify isoleucine (Ile) and methionine (Met), separately. Each of the other purine-ending sense co-don sets (NNR) specifies a single amino acid in the universal genetic code. In bacteria and archaea, the cytidine derivatives, 2-lysylcytidine (L or lysidine) and 2-agmatinylcytidine (agm(2)C or agmatidine), respectively, are found at the first letter of the anticodon of tRNA(Ile) responsible for AUA codons. These modifications prevent base pairing with G of the third letter of AUG codon, and enable tRNA(Ile) to decipher AUA codon specifically. In addition, these modifications confer a charging ability of tRNA(Ile) with Ile. Despite their similar chemical structures, L and agm(2)C are synthesized by distinctive mechanisms and catalyzed by different classes of enzymes, implying that the analogous decoding systems for AUA codons were established by convergent evolution after the phylogenic split between bacteria and archaea-eukaryotes lineages following divergence from the last universal common ancestor (LUCA).