Escherichia coli mutants deficient in the aspartate and aromatic amino acid aminotransferases

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Abstract
Two new mutations are described which, together, eliminate essentially all the aminotransferase activity required for de novo biosynthesis of tyrosine, phenylalanine and aspartic acid in a K-12 strain of E. coli. One mutation, designated tyrB, lies at .apprx. 80 min on the E. coli map and inactivates the tyrosine-repressible tyrosine/phenylalanine aminotransferase. The 2nd mutation, aspC, maps at .apprx. 20 min and inactivates a nonrepressible aspartate aminotransferase that also has activity on the aromatic amino acids. In ilvE- strains, which lack the branched-chain amino acid aminotransferase, the presence of the tyrosine-repressible aminotransferase or the aspartate aminotransferase is sufficient for growth in the absence of exogenous tyrosine, phenylalanine or aspartate; the tyrosine-repressible enzyme is also active in leucine biosynthesis. The ilvE gene product alone can reverse a phenylalanine requirement. Biochemical studies on extracts of strains carrying combinations of these aminotransferase mutations confirm the existence of 2 distinct enzymes with overlapping specificities for the .alpha.-keto acid analogs of tyrosine, phenylalanine and aspartate. These enzymes can be distinguished by electrophoretic mobilities, by kinetic parameters using various substrates and by a difference in tyrosine repressibility. In extracts of an ilvE- tyrB- aspC- triple mutant, no aminotransferase activity for the .alpha.-keto acids of tyrosine, phenylalanine or aspartate could be detected.