STRUCTURE, FUNCTION AND EVOLUTION OF SERYL-TRANSFER-RNA SYNTHETASES -IMPLICATIONS FOR THE EVOLUTION OF AMINOACYL-TRANSFER-RNA SYNTHETASES AND THE GENETIC-CODE

Two aspects of the evolution of aminoacyl-tRNA synthetases are discuss ed. Firstly, using recent crystal structure information on seryl-tRNA synthetase and its substrate complexes, the coevolution of the mode of recognition between seryl-tRNA synthetase and tRNA(ser) in different organisms is reviewed. Secondly, using sequence alignments and phyloge netic trees, the early evolution of class 2 aminoacyl-tRNA synthetases is traced. Arguments are presented to suggest that synthetases are no t the oldest of protein enzymes, but survived as RNA enzymes during th e early period of the evolution of protein catalysts. In this view, th e relatedness of the current synthetases, as evidenced by the division into two classes with their associated subclasses, reflects the repla cement of RNA synthetases by protein synthetases. This process would h ave been triggered by the acquisition of tRNA 3' end charging activity by early proteins capable of activating small molecules (e.g., amino acids) with ATP. If these arguments are correct, the genetic code was essentially frozen before the protein synthetases that we know today c ame into existence.