The Divergence in Reactivity of Aminoacyl Transfer Ribonucleic Acid Synthetases of Escherichia coli with Hydroxylamine

Fourteen partially purified Escherichia coli aminoacyl transfer ribonucleic acid synthetases were tested for their rates of amino acid hydroxamate formation relative to the rates of aminoacyl-tRNA formation. Three groups of reactivity were observed: tyrosyl-, phenylalanyl-, tryptophanyl-, and methionyl-tRNA synthetases catalyze hydroxamate formation at rates comparable to those for aminoacyl-tRNA formation; with lysyl-, alanyl-, isoleucyl-, glycyl-, cysteinyl-, leucyl-, seryl-, and valyl-tRNA synthetases, the relative rates of hydroxamate formation progressively decrease; and glutaminyl- and threonyl-tRNA synthetases do not catalyze hydroxamate formation. In an unfractionated enzyme extract, threonine hydroxamate forms, but this is totally accounted for by the mistaken activation of threonine by the valyl-tRNA synthetase. Purified threonyl-tRNA synthetase catalyzes hydroxamate formation only when tRNA is present, by catalyzing the formation of threonyl-tRNA, which reacts nonenzymatically with hydroxylamine. The mammalian threonyl-tRNA synthetase also fails to catalyze formation of threonine hydroxamate from amino acid and ATP.