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

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Abstract
The nucleotide sequences of the regulatory regions of 2 amino acid transport operons from Salmonella typhimurium were determined: 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 was 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 mechanisms. Three interesting features, present in both of these sequences, may be of importance in the regulation of these and other operons. These features are 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 in regulating expression of these and other operons in response to N availability. Mirror symmetry as a structure for DNA-protein interaction sites has not been previously proposed.