Poly-tRNA Theory on the Origin and Evolution of mRNA and Genetic Codes: Evolution from Tandem tRNA-Repeats to Primitive mRNAs Encoding F0-ATPase a Subunit and Glycyl-tRNA Synthetase

Koji Ohnishi

Department of Biology, Faculty of Science, Niigata University,
Ikarashi-2, Niigata 950-21, Japan.

The Bacillus subtilis trrnD operon has a structure of 5'[16S rRNA-23S rRNA-5S rRNA(tRNA)] 3'. The tRNA cluster in this operon includes 16 tandemly repeated tRNA genes (denoted by "poly-tRNA structure"), in which ordering of amino acid (aa) specificities of these tRNA is "NSEVMD FT YWHQ GCLL". An ancient "trrnD-peptide" possessing this aa sequence was hypothesized, and protein sequence regions similar to trrnD-peptide were searched for from PIR Proein Sequence Database. The aa's 139-156 in the E. coli Gly-tRNA synthetase (GlyRS) alpha subunit was found to be most similar to this peptide.

Further analysis revealed that not only the GlyRS gene encoding GlyRS alpha, but also the a gene of Synechococcus 6301 encoding F-ATPase a subunit, are bothtrue homologues of the BSU trrnD poly-tRNA region. These findings strongly support the recently proposed "poly-tRNA theory" (Ohnishi, 1993) on the origin of mRNA and genetic codes. Thus it has now been concluded that the trrnD poly-tRNA region is a relic of a most primitive RNA molecule capable of synthesizing a trrnD-peptide-like primitive peptide in early life. The most paradoxical problem on the origin of genetic codes seems to have been basically solved from the aspect of poly-tRNA theory.