THE AMINOACYL-tRNA SYNTHETASE FAMILY: AN EVOLUTIONARY VIEW OF THEIR STRUCTURAL ORGANIZATION

The aminoacyl-tRNA synthetases are a family of ubiquitous enzymes that function at an essential step in the translation of the genetic information. These enzymes are responsible for the accurate esterification of amino acids to the corresponding tRNA species. From an evolutionary point of view, this family of twenty enzymes has long been described as a class of proteins exhibiting similar catalytic functions but a puzzling structural diversity. The early studies revealed a large diversity in quaternary structures and in subunit molecular weights. In Escherichia coli, aminoacyl-tRNA synthetases have polypeptide chains ranging from 35 to 108 kDa, and are monomers, dimers or tetramers (Schimmel and Soll, 1979). Whereas these large differences argued against a unifying scheme for their structural organization, it was believed that an extensive relatedness should prevail in the aminoacyl-tRNA synthetase family. In particular, assuming that the primitive system was composed of fewer amino acids and activating enzymes, it was supposed that new synthetases arose through duplications and mutations in the genes of a restricted set of ancestral enzymes. This assumption led Orgel (1968) to ask the pertinent question: ’Is there amino acid sequence homology in the activating enzymes suggesting the course of specialization of these proteins?’