In animal mitochondria, six codons have been known as nonuniversal genetic codes, which vary in the course of animal evolution. They are UGA (termination codon in the universal genetic code changes to Trp codon in all animal mitochondria), AUA (Ile to Met in most metazoan mitochondria), AAA (Lys to Asn in echinoderm and some platyhelminth mitochondria), AGA/AGG (Arg to Ser in most invertebrate, Arg to Gly in tunicate, and Arg to termination in vertebrate mitochondria), and UAA (termination to Tyr in a planaria and a nematode mitochondria, but conclusive evidence is lacking in this case). We have elucidated that the anticodons of tRNAs deciphering these nonuniversal codons ( for UGA, for AUA, for AAA, and and for AGA/AGG) are all modified; has 5-carboxymethylaminomethyluridine or 5-taurinomethyluridine, has 5-formylcytidine or 5-taurinomethyluridine, has 7-methylguanosine and has 5-taurinomethyluridine in their anticodon wobble position, and has pseudouridine in the anticodon second position. This review aims to clarify the structural relationship between these nonuniversal codons and the corresponding tRNA anticodons including modified nucleosides and to speculate on the possible mechanisms for explaining the evolutional changes of these nonuniversal codons in the course of animal evolution. 1. Introduction Up to now six codons have been known which are deciphered by the corresponding tRNAs as amino acids different from those assigned by the universal genetic code in animal mitochondria (Figure 1) [1]. UGA termination codon in the universal genetic code is deciphered to Trp in all animal mitochondria, AUA Ile to Met in most metazoan except echinoderm, planarian, cnidarian, placozoan and poriferan mitochondria, AAA Lys to Asn in echinoderm and some platyhelminth mitochondria, and AGA/AGG Arg to Ser in most invertebrate mitochondria, Gly in tunicate (urochordata) mitochondria, and termination codon in vertebrate mitochondria. UAA termination codon was assumed to be a Tyr codon in a planaria [2] and a nematode mitochondria [3], but there is neither structural information on mt that decodes the UAA codon, nor information about the mitochondrial (mt) release factor relevant to this phenomenon. Thus, this issue is no more discussed here. Figure 1: Universal genetic code (inside the box) and variations in animal mt genetic code (outside). Term: termination codon. The codon-amino acid correspondence was first deduced by comparison of mt DNA sequence containing the codon with amino acid sequence of the corresponding protein [4]. Since mt proteins exist in a
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