The Trp repressor of Escherichia coli coordinately regulates the expression of the trp, aroH, and trpR operons in response to the intracellular levels of L-tryptophan (Cohen and Jacob 1959;Brown 1968;Rose et al. 1973;Gunsalus and Yanofsky 1980). The trpR operon, located at 99.5 min, codes for the Trp aporepressor and is autoregulated (Morse and Yanofsky 1969;Gunsalus and Yanofsky 1980;Kelley and Yanofsky 1982;Bachman 1983;Gunsalus et al. 1986). The aroH operon, located at 37 min, encodes one of the three isozymes of 3-deoxy-D-arabino-heptulosonic acid-7-phosphate synthetase (DAHP). This enzyme catalyzes the first step in the common pathway of aromatic amino acid biosynthesis (Brown 1968;Pittard et al. 1969;Zurawski et al. 1981). The third operon, trp, is located at 27 min on the E. coli genetic map and encodes the enzymes that carry out the final steps of L-tryptophan biosynthesis from the cellular intermediate, chorismic acid (Yanofsky 1971).The native form of the Trp aporepressor is a dimer composed of identical subunits of 108 amino acids (Gunsalus and Yanofsky 1980;Joachimiak et al. 1983;Arvidson et al. 1986). Binding of the corepressor, L-tryptophan increases the affinity of Trp repressor for operator DNA, leading to the formation of the ternary repressor DNA complex (Rose et al. 1973). The binding of Trp repressor to its operators inhibits the initiation of transcription of the trpR, aroH, and trp operon messages by preventing RNA polymerase from binding to the promoters (Oppenheim et al. 1980).The physical location of the trp operator was previously defined by deletion studies (Bennett et al. 1976) and by the isolation of constitutive mutations within the trp operator {Bennett and Yanofsky 1978). These operator constitutive mutations map at positions -16, -15, -7, and -6 in the trp promoter sequence. The DNA sequences of the trpR and aroH operons reveal sites that exhibit significant sequence homology to the trp operator (Gunsalus and Yanofsky 1980; Singelton et al. 1980; Zurawski et al. 19811. In Figure 1, the sequences of the three operators are aligned to show max- • .• . .• .• ::::~ -20 .....