EVOLUTION OF TRANSFER-RNA RECOGNITION SYSTEMS AND TRANSFER-RNA GENE-SEQUENCES

The aminoacylation of tRNAs by the aminoacyl-tRNA synthetases recapitu lates the genetic code by dictating the association between amino acid s and tRNA anticodons. The sequences of tRNAs were analyzed to investi gate the nature of primordial recognition systems and to make inferenc es about the evolution of tRNA gene sequences and the evolution of the genetic code. Evidence is presented that primordial synthetases recog nized acceptor stem nucleotides prior to the establishment of the thre e major phylogenetic lineages. However, acceptor stem sequences probab ly did not achieve a level of sequence diversity sufficient to faithfu lly specify the anticodon assignments of all 20 amino acids. This puta tive bottleneck in the evolution of the genetic code may have been all eviated by the advent of anticodon recognition. A phylogenetic analysi s of tRNA gene sequences from the deep Archaea revealed groups that ar e united by sequence motifs which are located within a region of the t RNA that is involved in determining its tertiary structure. An associa tion between the third anticodon nucleotide (N36) and these sequence m otifs suggests that a tRNA-like structure existed close to the time th at amino acid-anticodon assignments were being established. The sequen ce analysis also revealed that tRNA genes may evolve by anticodon muta tions that recruit tRNAs from one isoaccepting group to another. Thus tRNA gene evolution may not always be monophyletic with respect to eac h isoaccepting group.