Evolution from possible primitive tRNA-viroids to early poly-tRNA-derived mRNAs: a new approach from the poly-tRNA theory.

Poly-tRNA theory have revealed that the tRNA gene-clusters in the Bacillus subtilis trrnD- and rrnB-operons are relics of early peptide-synthesizing RNA apparatus. The trrnD-type and rrnB-type poly-tRNA models were re-analyzed by using recent databases. The results elucidated that the 16 amino acid (aa)-trrnD- and the 21 aa-rrnB-peptides (whose aa sequences are in the order of aa specificities of tRNAs in the respective tRNA cluster) are really relics of earliest peptides encoded by most primitive mRNAs, trrnD-mRNA and rrnB-mRNA, which are homologous to tRNA(Gly) and tRNA(His), respectively. Genes encoding various protein superfamilies (including pgtB protein, glycyl-tRNA synthetase alpha, C-type lectin, F1-ATP-synthase gamma, etc) were concluded to have derived from tRNA(Gly)-tRNA(Cys)-tRNA(Leu) region (including trrnD-mRNA region) in the trrnD-poly-tRNA. Genes for another group of protein superfamilies (including adenylate kinase, glyceraldehyde-3-phosphate dehydrogenase, helix-turn-helix DNA-binding domains, etc.) were found to have derived rrnB-mRNA, which is most plausibly homologous to a region containing the tRNA(His) of the rrnB-poly-tRNA. Thus Proto-tRNA((Gly)) reconstructed from tRNA(Gly) and other tRNAs strongly suggested that proto-tRNA was most plausibly a viroid-like possibly self-cleavable replicable ribozyme possessing a possible hammerhead-like structure.