Study of the evolution of the genetic code by comparing the structural and catalytic properties of the aminoacyl-tRNA synthetases

Proteins of the primitive cells probably contained only a few of the 20 amino acid species presently incorporated into proteins. The 20 corresponding species of aminoacyl-tRNA synthetases found in most cells could result from the divergent evolution of the genes of one or a few primitive aminoacyl-tRNA synthetases. A comparison of the structural and catalytic properties of the 20 aminoacyl-tRNA synthetases should help to determine the steps of this evolution, which is also that of the genetic code. First, the very diverse quaternary structures of these synthetases are not generally correlated with the evolutionary linkages suggested by many theories of the evolution of the genetic code. The results discussed here indicate that these quaternary structures are more related to the various regulatory functions of the aminoacyl-tRNA synthetases than to their basic function which is the specific aminoacylation of tRNAs. Secondly, a comparison of the catalytic peculiarities of these enzymes appears to be useful for understanding the evolution of the genetic code; for instance, the requirement for the presence of tRNA in the activation of glutamate, glutamine, and arginine by the corresponding aminoacyl-tRNA synthetases, and the absence, in some Gram-positive bacteria, of a glutaminyl-tRNA synthetase, which is replaced by a less specific glutamyl-tRNA synthetase and an amido-transferase of Glu-tRNAGln, suggest that these three aminoacyl-tRNA synthetases specific for amino acids of the glutamate family have a common ancestor. Finally, the comparison of the sequences of the structural genes of these enzymes is a promising approach for studying their evolution and that of the genetic code.