Internal promoter of the tryptophan operon of Escherichia coli is located in a structural gene

The constitutive low-efficiency promoter site (P₂) near the middle of the tryptophan operon of Escherichia coli has been mapped by analysis of short deletions internal to the trp operon. Comparison of deletions which remove this internal promoter with those which retain it show that P₂ is located within trpD, the region coding for phosphoribosyl anthranilate transferase. P₂ maps near the operator-distal end of trpD, on the operator-proximal side of two trpD point mutants. Comparisons of strains with and without the P₂ site indicate that initiations at this promoter are responsible for synthesis of 80% of the trpC, trpB and trp A polypeptides present in repressed cells.