Macromolecular Complex of Aminoacyl‐tRNA Synthetases from Sheep Liver

Both the tRNA aminoacylation and aminoacid-dependent ATP-PPi exchange activities of monomeric trypsin modified methionyl-tRNA synthetase from sheep liver are lost upon incubation with oxidized initiator tRNAmet. The inactivation, which reflects the formation of a Schiff's base between the 5′-terminal adenosine of tRNA and a lysine within the catalytic site of the enzyme, is accompanied by the covalent attachment of one tRNA molecule per enzyme molecule. The affinity labeling method is applied to the sheep liver complex of Mt106 carrying seven aminoacyl-tRNA synthetase activities, from which the monomeric trypsin-modified methinnyl-tRNA synthetase (Mt 68000) was derived. Upon incubation with oxidized initiator tRNAMet, the methionyl-tRNA synthetase activity of the complex is lost. Of the eleven polypeptidechains composing the high-molecular-weight complex, only one polypeptide chain with Mt 103000 reacts with the modified tRNAMet The blocking by periodate-treated tRNA of the methionyl-tRNA synthetase activity in the complex has no effect on the other aminoacyl-tRNA synthetase activities. This strongly argues in favor of the independent parallel functioning of the seven aminoacyl-tRNA syn- thetases associated in a high-molecular-weight complex.