Regulatory regions of two transport operons under nitrogen control: nucleotide sequences.

We have determined the nucleotide sequences of the regulatory regions from two amino acid transport operons from Salmonella typhimurium: dhuA, which regulates the histidine transport operon, and argTr, which regulates argT, the gene encoding the lysine-arginine-ornithine-binding protein, LAO. The promoter for the histidine transport operon has been identified from the sequence change in the promoter-up mutation dhuA1. Neither regulatory region has any of the features typical of the regulatory regions of the amino acid biosynthetic operons, indicating that regulation of at least these transport genes does not involve a transcription attenuation mechanism. We have identified three interesting features, present in both of these sequences, which may be of importance in the regulation of these and other operons: a "stem-loop-foot" structure, a region of specific homology, and a mirror symmetry. The region of mirror symmetry may be a protein recognition site important is regulating expression of these and other operons in response to nitrogen availability. Mirror symmetry as a structure for DNA-protein interaction sites has not been proposed previously.