Feedback Inhibition of the DAHP Synthetases by t RNA

Summary. The use of tyrosine inhibited and phenylalanine inhibited mutants in yeast makes possible the study of the properties of the two 3 deoxy-D-arabino-heptulosonie-aeid7-phosphate synthetases separately. The measurement of the activity of these two enzymes revealed the presence of endogenous inhibitors of the reaction in the crude extracts. These inhibitors are non dialysable, thermostable and sensitive to an RNAse treatment. The use of purified tRNAs showed that the phenylalanine sensitive enzyme is inhibited by both charged and uncharged phenylalanyl tRNA whereas the tyrosine sensitive enzyme is inhibited only by charged tyrosyl tRNA whereas the tyrosine sensitive enzyme is inhibited only by charged tyrosyl tRNA. Both enzymes are also inhibited by the free amino acids, i.e. respectively tyrosine and phenylalanine.