Aminoacyl-tRNA synthetases, the genetic code, and the evolutionary process

The aminoacyl-tRNA synthetases (AARSs) and their relationship to the geneti c code are examined from the evolutionary perspective. Despite a loose corr elation between codon assignments and AARS evolutionary relationships, the code is fnr too highly structured to have been ordered merely through the e volutionary wanderings of these enzymes. Nevertheless, the AARSs are very i nformative about the evolutionary process. Examination of the phylogenetic trees for each of the AARSs reveals the following. (i) Their evolutionary r elationships mostly conform to established organismal phylogeny: a strong d istinction exists between bacterial- and archaeal-type AARSs. (ii) Although the evolutionary profiles of the individual AARSs might be expected to be similar in general respects, they are not. It is argued that these differen ces in profiles reflect the stages in the evolutionary process when the tax onomic distributions of the individual AARSs became fixed not the nature of the individual enzymes. (iii) Horizontal transfer of AARS genes between Ba cteria and Archaea is asymmetric: transfer of archaeal AARSs to the Bacteri a is more prevalent than the reverse, which is seen only for the "gemini gr oup." (iv) The most far-ranging transfers of AARS genes have tended to occu r in the distant evolutionary past, before or during formation of the prima ry organismal domains. These findings ale also used to refine the theory th at at the evolutionary stage represented by the root of the universal phylo genetic hse, cells were far more primitive than their modern counterparts a nd thus exchanged genetic material in far less restricted ways, in effect e volving in a communal sense.