The nucleotide sequence of the region containing the promoter-operator for the aroP gene was determined. The start site of aroP transcription was identified by using S1 nuclease mapping and primer extension techniques. Examination of the nucleotide sequence revealed the presence of two "TYR R" boxes which are similar to those identified in the regulatory regions of other genes in the tyrR regulon. Bisulfite-induced aroP operator-constitutive mutants were analyzed, and the base-pair changes responsible for alterations in aroP regulation were located within these boxes.