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 (agm2C or agmatidine), respectively, are found at the first letter of the anticodon of tRNAIle responsible for AUA codons. These modifications prevent base pairing with G of the third letter of AUG codon, and enable tRNAIle to decipher AUA codon specifically. In addition, these modifications confer a charging ability of tRNAIle with Ile. Despite their similar chemical structures, L and agm2C 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).

对生物体来说，解读AUA密码子是一项艰巨的任务，因为AUA和AUG分别指定异亮氨酸（Ile）和甲硫氨酸（Met）。在通用遗传密码中，其他每个以嘌呤结尾的有义密码子组（NNR）都指定一种单一的氨基酸。在细菌和古菌中，分别在负责AUA密码子的tRNAIle的反密码子的第一个碱基位置发现胞苷衍生物，2 - 赖氨酰胞苷（L或赖胞苷）和2 - 胍基丁胺酰胞苷（agm2C或胍基丁胺苷）。这些修饰阻止了与AUG密码子第三个碱基G的碱基配对，并使tRNAIle能够特异性地解读AUA密码子。此外，这些修饰赋予tRNAIle携带异亮氨酸的能力。尽管L和agm2C化学结构相似，但它们是通过不同的机制合成的，并由不同类别的酶催化，这意味着在细菌与古菌 - 真核生物谱系在从最后一个共同祖先（LUCA）分化之后的系统发育分支之后，AUA密码子的类似解码系统是通过趋同进化建立的。