Mutant strains of Escherichia coli have been isolated in which the synthesis of 3deoxy-D-arabinoheptulosonic acid 7-phosphate (DAHP) synthetase (phe) is derepressed, in addition to those enzymes of tyrosine biosynthesis previously shown to be controlled by the gene tyrR. The major enzyme of the terminal pathway of phenylalanine biosynthesis chorismate mutase-prephenate dehydratase is not derepressed in these strains. Genetic analysis of the mutants shows that the mutation or mutations causing derepression map close to previously reported tyrR mutations. A study of one of the mutations has shown it to be recessive to the wild-type allele in a diploid strain. It is proposed that the tyrR gene product is involved in the regulation of the synthesis of DAHP synthetase (phe) as well as the synthesis of DAHP synthetase (tyr), chorismate mutase-prephenate dehydrogenase, and transaminase A.The first reaction of aromatic biosynthesis, the condensation of erythrose-4-phosphate and phosphoenolpyruvate to form 3-deoxy-D-arabinoheptulosonic acid 7-phosphate (DAHP) is carried out in Escherichia coli by three isoenzymes, the synthesis and activity of each of these isoenzymes being controlled by tyrosine, phenylalanine, and tryptophan, respectively (4, 10, 17). These amino acids also control the synthesis of one or more enzymes in each of the terminal pathways from chorismic acid to tyrosine, phenylalanine, and tryptophan (10). In the case of tyrosine biosynthesis, the structural genes for DAHP synthetase (tyr) and chorismate mutaseprephenate dehydrogenase (a key enzyme in the tyrosine pathway) form a single operon, the expression of which is controlled by a regulator gene tyrR (Mattern and Pittard, in press). In tryptophan biosynthesis, the structural gene for DAHP synthetase (trp) is separated from the structural genes of the trp operon; however, all three are subject to control by the regulator gene trpR (5, 14; Camakaris and Pittard, in press).The genes for DAHP synthetase (phe) and chorismate mutase-prephenate dehydratase are also separated from each other on the chromosome; although operator mutants controlling the expression of the gene for chorismate mutase-prephenate dehydratase have been described (Im and Pittard, in press), there have as yet been no reports of mutations affecting the synthesis of DAHP synthetase (phe). It is the purpose of this paper to describe the isolation and characterization of mutants derepressed for the synthesis of DAHP synthetase (phe). Although the method that was used to isolate these mutants was expected to select for strains derepressed for both chorismate mutase-prephenate dehydratase and DAHP synthetase (phe), chorismate mutaseprephenate dehydratase is not derepressed in these mutants. However, the observation that the tyrosine biosynthetic enzymes are also derepressed in these mutants suggests the possibility of some relationship in the control of the two pathways.In an accompanying paper (4a), Brown and So-400 on July 15, 2020 by guest