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Journal of Nucleic Acids
Volume 2011, Article ID 623095, 12 pages
http://dx.doi.org/10.4061/2011/623095
Review Article

tRNA Modification and Genetic Code Variations in Animal Mitochondria

1Department of Molecular Biology, School of Life Sciences, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
2Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-Ku, Tokyo 113-8657, Japan

Received 31 May 2011; Accepted 4 July 2011

Academic Editor: Daisuke Miyoshi

Copyright © 2011 Kimitsuna Watanabe and Shin-ichi Yokobori. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

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 ( t R N A T r p for UGA, t R N A M e t for AUA, t R N A A s n for AAA, and t R N A S e r and t R N A G l y for AGA/AGG) are all modified; t R N A T r p has 5-carboxymethylaminomethyluridine or 5-taurinomethyluridine, t R N A M e t has 5-formylcytidine or 5-taurinomethyluridine, t R N A S e r has 7-methylguanosine and t R N A G l y has 5-taurinomethyluridine in their anticodon wobble position, and t R N A A s n 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.