The threonyl-tRNA synthetase of two wild-type Escherichia coli K12 strains and of two borrelidin-resistant mutants thereof has been purified about 100-fold. A neutralizing antiserum has been prepared against one of the wild-type threonyl-tRNA synthetases. The comparative characterization of the threonyl-tRNA synthetases of the two wild strains and the two borrelidin-resistant mutants revealed that the threonyl-tRNA synthetase of one borrelidinresistant mutant (K12B borr 2) exhibits a lowered K , value for threonine and ATP and an increased Xi value for borrelidin with respect to threonine. Furthermore the activity of the threonyl-tRNA synthetase of this mutant k more sensitive to higher p H values, is more rapidly heat-inactivated and shows a different shape of its antiserum-neutralization curve than the threonyl-tRNA synthetase of the parental strain (K12B). The threonyl-tRNA synthetase of the other borrelidin-resistant mutant (K12B borr 3) was investigated in the same way and it did not show any difference to the threonyl-tRNA synthetase of the parental strain, except that the specific activity of the enzyme in crude extracts of this mutant is 5 times as high as the corresponding value in crude extracts of K12B.From these data it is concluded that borrelidin resistance in K12B borr-2 is due to a structurally altered threonyl-tRNA synthetase (structural mutant) whereas in K12B borr-3 it is due to the constitutive increase of the threonyl-tRNA synthetase level by a factor of five (regulatory mutant).The inhibition of the threonyl-tRNA synthetase by borrelidin with respect to threonine was noncompetitive in all four cases. I n order to observe a considerable heat inactivation of the threonyl-tRNA synthetase within 10 min a temperature of 60 "C was necessary. The heatinactivation process was not affected by the presence of threonine, but it was hastened by the presence of ATP or tRNA.

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