Mutational uncoupling of the transcriptional activation function of the TyrR protein of Escherichia coli K-12 from the repression function.

The tyrosine repressor (TyrR) protein of Escherichia coli can function either as a transcriptional enhancer or as a repressor. The structural basis for these opposite effects was analyzed in specific tyrR deletion mutants constructed in vitro. The functional behavior of the mutant TyrR proteins was evaluated in vivo by using single-copy lacZ reporter systems based on the mtr promoter (10-fold activation by wild-type TyrR protein, mediated by phenylalanine or tyrosine) or the aroF promoter (over 20-fold repression by wild-type TyrR protein, mediated by tyrosine). A mutant TyrR protein lacking amino acids 2 to 9 was completely devoid of transcriptional activation function. Five additional mutant TyrR proteins lacking progressively greater numbers of N-terminal amino acids were likewise activation defective. The mutant TyrR proteins lacking amino acid residues 2 to 9 or 2 to 19 were essentially identical to the wild-type TyrR protein in their ability to repress the aroF promoter. Three other TyrR mutant proteins, lacking up to 143 amino acid residues from the N-terminal end of the protein, retained the ability to repress the aroF promoter, to different extents, in a tyrosine-dependent manner.