Affinities of tight-binding lactose repressors for wild-type and pseudo-operators.

Five tight-binding (Itb) mutants of the Escherichia coli lactose (lac) repressor have been characterized with regard to their non-specific affinity for DNA and their specific affinity for the wild-type operator and several sequence-altered (pseudo-) operators. Repressor-operator association rates were determined in the presence or absence of competitor DNA, dissociation rates of repressor from various DNA fragments were measured, and equilibrium competition for repressor binding was examined for several pseudo-operator DNAs. The mutant repressors exhibited increased non-specific affinity for DNA, and variable increases in affinity for sequence-altered operators. The known positions of amino acid substitutions for three of these Itb repressors support suggestions that residues 51 to 64 are important for operator recognition in addition to residues 1 to 50.