A 4509 base-pair DNA fragment containing the phenylalanine and tyrosine operons of Escherichia coli K12 has been sequenced, and the pattern of transcription of these operons examined by S1 mapping, primer extension and galK fusion analyses. The phe operon consists of promoter, operator, leader region containing the phe attenuator and the pheA gene encoding chorismate mutase/prephenate dehydratase. The tyr operon consists of promoter, operator, a short leader region without an attenuator, and two structural genes aroF and tyrA encoding the tyrosine-sensitive isoenzyme of 3-deoxy-D-arabinoheptulosonate-7-phosphate (DAHP) synthetase and chorismate mutase/prephenate dehydrogenase, respectively. A bidirectional transcription terminator occurs between the two operons. The predicted amino acid sequences of chorismate mutase/prephenate dehydrogenase and chorismate mutase/prephenate dehydratase are homologous at their N termini, while the tyrosine-sensitive isoenzyme of DAHP synthetase is closely homologous to the phenylalanine-sensitive isoenzyme encoded by aroG.