The Split Nanoarchaeum equitans tRNA Genes Are the Ancestral Genes from Which Modern tRNA Genes Evolved

It has been convincingly shown that, based on the following arguments, the split genes coding for the 5' and 3' halves of the tRNA molecule are the plesiomorphic (ancestral) forms ofmodern tRNA genes:1,15

i. The introns of tRNA genes are normally located 3' to po-sition-37 on the tRNA, which is the same position where

Figure 13.2. Model for the origin of the tRNA molecule. For details see Di Giulio.15-17 ANT stands for anticodon and ID the nucleotides determining the identity of the tRNA in its recognition by its cognate aminoacyl-tRNA synthetase. The triangle indicates the normal position of intron in tRNA genes.16,19

Figure 13.2. Model for the origin of the tRNA molecule. For details see Di Giulio.15-17 ANT stands for anticodon and ID the nucleotides determining the identity of the tRNA in its recognition by its cognate aminoacyl-tRNA synthetase. The triangle indicates the normal position of intron in tRNA genes.16,19

the N. equitans half genes for the 5' and 3' halves of tRNA are interrupted.1,10,15 This implies that the tRNA half genes and the tRNA genes carrying an intronic insertion must represent different stages ofthe same evolutionary sequence (Fig. 13.3A; ref. 1, 15), because the probability of both of these scissions occurring 3' to position-37 on account of chance is extremely low. If the tRNA half genes and the tRNA genes with introns represent different stages of the same evolutionary sequence, then continuous tRNA genes without any intron must be the final stage of this sequence of tRNA evolution. Thus the sequence of tRNA evolution went through the distinct Ancestral, Intermediate and Current stages shown in Figure 13.3A. This sequence in time, by bringing the two scattered split tRNA genes of the Ancestral Stage at first close to one another separated only by an intron as in the Intermediate Stage and finally continuous with one another as in the Current Stage, has progressively reduced the likelihood of committing any errors during the construction of the cloverleaf tRNA molecule.1,15 In contrast, based on the alternative view depicted in Figure 13.3B, the evolutionary sequence, leading from continuous tRNA genes to tRNA genes with introns and finally to scattered split genes, would result in error enhancement rather than error reduction in the course of evolution, which is unreasonable. ii. The absolute gene frequencies found among extant organisms for these three stages also strongly support the evolutionary sequence in Figure 13.3A over that in Figure 13.3B. There are many more species of organisms with continuous tRNA genes than tRNA genes with introns, which in turn far exceed the number oforganisms (so far only N. equitans) with split tRNA genes. This is entirely consistent with the relic status assigned to split tRNA genes in Figure 13.3A. In contrast, Figure 13.3B would regard continuous tRNA genes, the majority of tRNA genes in the living world, as evolutionary relics. The fact that relics are as a rule very rare therefore contradicts continuous ancestral tRNA genes and supports split ancestral tRNA genes.1,15

Consequently, the half genes encoding the 5' and 3' halves of tRNA molecules in N. equitans represent the ancestral form oftRNA genes from which modern tRNA genes might have evolved,1,15 They are also the minigenes predicted by the exon theory of genes.

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