Decoding Mechanism of Non-universal Genetic Codes in Loligo bleekeri Mitochondria

摘要

Non-universal genetic codes are frequently found in animal mitochondrial decoding systems. In squid mitochondria, four codons deviate from the universal genetic code, namely AUA, UGA, and AGA/AGG (AGR) for Met, Trp, and Ser, respectively. To understand the molecular basis for establishing the non-universal genetic code, we isolated and analyzed five mitochondrial tRNAs from a squid, Loligo bleekeri. Primary structures of the isolated tRNAs, including their post-transcriptional modifications, were analyzed by mass spectrometry. tRNA^Met(AUR) possessed an unmodified cytidine at the first position of the anticodon, suggesting that the AUA codon is deciphered by CAU anticodon via non-canonical A-C pairing. We identified 5-taurinomethyluridine (τm⁵U) at the first position of the anticodon in tRNA^Trp(UGR). τm⁵U enables tRNA^Trp to decipher UGR codons as Trp. In addition, 5-taurinomethyl-2-thiouridine (τm⁵s²U) was found in mitochondrial tRNAs for Leu(UUR) and Lys in L. bleekeri. This is the first discovery of τm⁵U and τm⁵s²U in molluscan mitochondrial tRNAs.