Expression of the mtr gene, which encodes a tryptophan-specific transport system in Escherichia coli K-12, is activated by the TyrR protein. Two TyrR protein binding sites (TYR R boxes) are positioned upstream of the -35 promoter region. Mutational and DNase protection studies indicate that TyrR protein binds preferentially to the TYR R box closest to the promoter, and this is essential for activation of gene expression. In the presence of tyrosine and ATP, a second TyrR molecule is able to cooperatively bind to the second box and cause a further increase in the level of activation
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