Mutant tryptophan aporepressors with altered specificities of corepressor recognition.

The Escherichia coli trpR gene encodes tryptophan aporepressor, which binds the corepressor ligand, L-tryptophan, to form an active repressor complex. The side chain of residue valine 58 of Trp aporepressor sits at the bottom of the corepressor (L-tryptophan) binding pocket. Mutant trpR genes encoding changes of Val58 to the other 19 naturally occurring amino acids were made. Each of the mutant proteins requires a higher intracellular concentration of tryptophan for activation of DNA binding than wild-type aporepressor. Whereas wild-type aporepressor is activated better by 5-methyltryptophan (5-MT) than by tryptophan, Ile58 and other mutant aporepressors prefer tryptophan to 5-MT as corepressor, and Ala58 and Gly58 prefer 5-MT much more strongly than wild-type aporepressor in vivo. These mutant aporepressors are the first examples of DNA-binding proteins with altered specificities of cofactor recognition.