The interaction of the trp repressor from Escherichia coli with a 20 bp fragment of DNA (CGTACTGATT.AATCAGTACG) corresponding to a mutant trp operator was studied by c.d. in the presence and absence of the co-repressor, L-tryptophan, and as a function of the concentration of K+ and Na+ ions. The affinity of the repressor for the mutant operator in the presence of tryptophan is about three orders of magnitude lower than the wild-type sequence. Binding in the absence of tryptophan is about 100-fold weaker than to the wild-type. The dependence of the dissociation constant on the concentration of K+ or Na+ is weak [d(log Ks)/d(log[M+]) = 2.5], and independent of the cation, indicating that electrostatic interactions are not as important for this repressor as for others.
