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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.
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Hirsh et al. (1968) studied this question.
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