Genetic code variations in mitochondria: tRNA as a major determinant of genetic code plasticity.

Characteristic features of tRNA such as the anticodon sequence and modified nucleotides in the anticodon loop are thought to be crucial effectors for promoting or restricting codon reassignment. Our recent findings on basepairing rules between anticodon and codon in various metazoan mitochondria suggest that the complete loss of a codon is not necessarily essential for codon reassignment to take place. We postulate that a possible competition between two tRNAs with cognate anticodon sequences towards the relevant codon to be varied has a potential role in codon reassignment. Our proposition can be viewed as an expanded version of the codon capture theory proposed by Osawa and Jukes (J Mol Evol 28: 271-278, 1989).