The hypothesis that the tRNA molecule may have originated from the assembly of two similar RNA hairpin structures is utilised to understand the evolutionary period in which this molecule originated. Consistent with the exon theory of genes is the observation that the introns in tRNA genes are found almost exclusively in the anticodon loop and "stitched together" the two halves of the molecule, which originally may have been simply two hairpin structures and which can still be observed in the three-dimensional structure of tRNAs. This theory therefore considers these hairpin structures as minigenes on which complex protein synthesis may have been achieved. This in turn leads to the belief that the organisation of the genetic code may have been determined by use of the hairpin structures but not the complete tRNA molecule. In view of this, it can be conjectured that tRNA molecules might have been assembled only after the establishment of the main phyletic lines. If this is all true, then the origin of the tRNA molecule might have been non-monophyletic, i.e. a tRNA specific for a certain amino acid might have been assembled in different phyletic lines with a second and different hairpin structure. This leads to the belief that tRNAs specific for different amino acids but belonging to the same phyletic line might have been more similar to one another than to tRNAs specific for the same amino acid but belonging to different phyletic lines. This prediction seems to be supported by phylogenetic analysis making major use of the bootstrap technique performed on the tRNA sequences and by analysis already existing in the literature which supports the non-monophyletic origin of the tRNA molecule. The main conclusion of this paper is that if the tRNA molecule was assembled in the main phyletic lines this would imply a still rapidly evolving translation apparatus which, in turn, seems to imply that the last universal common ancestor was a progenote.
Copyright 1999 Academic Press.