Synergism between the Trp repressor and Tyr repressor in repression of the aroL promoter of Escherichia coli K-12.

Computer analysis identified a potential Trp repressor operator 56 nucleotides downstream of the transcriptional start point of aroL, the gene that encodes shikimate kinase II. Tryptophan-dependent interaction of Trp repressor with this operator was demonstrated in vitro by means of a restriction endonuclease protection assay. Regulation of expression from the aroL promoter was evaluated with several genetically marked Escherichia coli strains by using a single-copy aroL-lacZ transcriptional-translational reporter system. The expression of aroL was repressed 6.9-fold by the Tyr repressor alone and 29-fold when both Tyr and Trp repressors were present. The Trp repressor had no effect on expression from the aroL promoter in the absence of the Tyr repressor. Possible mechanisms for Trp repressor-mediated repression, including cooperative interactions with the Tyr repressor, are discussed.