An evaluation of mitochondrial tRNA gene evolution and its relation to the genetic code.

Extensive sequence data on mitochondrial (mt) tRNAs give for the first time an opportunity to evaluate tRNA gene evolution in this organelle. Deductions from these gene structures relate to the evolution of tRNA genes in other cellular systems and to the origin of the genetic code. Mt tRNAs, in contrast to the prokaryotic nature of chloroplastic tRNA structure, can not at the present time be definitely related to either prokaryotic or eukaryotic tRNAs, probably because of a higher mutation rate in mitochondria. Fungal mt tRNAs having the same anticodon and function are generally similar enough to be considered homologous. Comparisons af all mt tRNA sequences contained in the same mitochondrion indicate that some tRNAs originated by duplication of a prototypic gene which, after divergence, led to tRNAs having different amino acid specificities. The deviant mt genetic code, although admittedly permitting a simpler decoding mechanism, is not useful in determining whether the origin of mitochondria had preceded or was derived from prokaryotes or eukaryotes, since the genetic code is variable even among mitochondria. Variants of the mt genetic code lead to speculation on the nature of the primordial code and its relation to the present "universal" code.