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Repression of the formation of biosynthetic enzymes by the ultimate end product of their action has been shown to be an important aspect of metabolic regulation in microorganisms.1 In Escherichia coli and Salmonella typhimuwium each of the last four steps in the synthesis of valine and isoleucine is catalyzed by the same enzymes.2 Repression of these enzymes presents a unique situation since two ultimate end products are formed by their action. In addition, the initial reaction leading to leucine biosynthesis involves an intermediate in the valine pathway.3 In this report we would like to describe experiments which indicate that valine, isoleucine, and leucine are required for the repression of the enzymes necessary for isoleucine and valine biosynthesis. In contrast to these results, the enzymes of the leucine pathway appear to be repressed only by leucine. Material7s and Method&.-Organisms: The following derivatives of S. typhimnurilum strain LT-2 were used: leit 130 ilva 224 which lacks enzymes 3 and 7 (Fig. 1) and therefore requires isoleucine, valine, and leucine; leu 124 which lacks the initial enzyme in leucine biosynthesis (Fig. 1, enzyme 6); and ilva A-8 which is an isoleucine-valine auxotroph deficient in enzyme 3 (Fig. l) anld which was kindly supplied by Dr. M. Demerec. A mutant derived from E. coli strain W, M4862-H5, Which has a partial block in enzyme 3 and lacks enzyme 5 (Fig. 1), was employed. It has an absolute requirement for isoleucine and valine and a partial requirement for leucine. Chemicals: a-Hydroxy-fl-carboxyisocaproate was isolated from culture filtrates of Neurospora crassa using the method of Gross et al.4 a,0-I)ihydroxyisovalerate was prepared according to the procedure of Sjolander et al.5 Other compounds were of reagent grade. Enzyme methods: For this study the following enzymes were examined: (1) L-threonine dealminase (Fig. 1, enzyme 1) Which is involved only in the formation of isoleucine; (2) dihydroxyacid dehydrase (Fig. 1, enzyme 4) which is necessary for the biosynthesis of isoleucine and valine and (3) a-hydroxy-_#-carboxyisocaproate decarboxylase (Fig. 1. enzyme 8) which is required

The nucleotide sequence of the DNA thought to contain the control region for the ilvGEDA operon in Escherichia coli has been determined by the Maxam-Gilbert procedure. The sequence includes a region that, upon transcription, would yield a leader transcript specifying a peptide 32 residues long. This putative peptide would contain four leucine, five isoleucine, and six valine residues. A model is proposed that correlates the multivalent control of the ilvGEDA operon with the extent to which this leader transcript is translated. In vitro transcription experiments yielded a transcript of about 183 nucelotides, compatible with the predictions of the model.