Interaction of the tight-binding I12-X86 lac repressor with non operator DNA: salt dependence of complex formation

The interaction of the wild-type lac repressor and its tight binding double mutant I12-X86 with a non operator-210 base pair-DNA fragment has been investigated using the nitrocellulose filter binding assay. While the affinity of the double mutant for this non specific DNA is increased as compared to that of the wild-type repressor, the number of ions released from the vicinity of the DNA upon complex formation is less important for the mutant than for the wild-type. These results demonstrate that the adaptation in the recognition surface of the repressor recently proposed by Mossing et al (J. Mol. Biol., 1985, 186, 295-305) in the case of an Oc mutant may be a more general phenomenon.