The non-monophyletic origin of the tRNA molecule

The hypothesis that the tRNA molecule may have originated from the assembly of two similar RNA hairpin structures is utilised to understand the evolut ionary period in which this molecule originated. Consistent with the exon t heory of genes is the observation that the introns in tRNA genes are found almost exclusively in the anticodon loop and "stitched together" the two ha lves of the molecule, which originally may have been simply two hairpin str uctures and which can still be observed in the three-dimensional structure of tRNAs. This theory therefore considers these hairpin structures as minig enes on which complex protein synthesis may have been achieved. This in tur n leads to the belief that the organisation of the genetic code may have be en determined by use of the hairpin structures but not the complete tRNA mo lecule. In view of this, it can be conjectured that tRNA molecules might ha ve 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 n on-monophyletic, i.e, a tRNA specific for a certain amino acid might have b een assembled in different phyletic lines with a second and different hairp in structure. This leads to the belief that tRNAs specific for different am ino acids but belonging to the same phyletic line might have been more simi lar to one another than to tRNAs specific for the same amino acid but belon ging 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 w hich supports the non-monophyletic origin of the tRNA molecule. The main co nclusion of this paper is that if the tRNA molecule was assembled in the ma in phyletic lines this would imply a still rapidly evolving translation app aratus which, in turn, seems to imply that the last universal common ancest or was a progenote. (C) 1999 Academic Press.