Tight-binding repressors of the lactose operon

Sixteen mutants which produce lactose repressors with enhanced operator affinities have been isolated. By deletion mapping, six of seven mutations mapped fall into a restricted region of the i gene which also is the location of some anomalous i^s (super-repressor) and some weak i^?d mutations (Pfahl et al., 1974). In vivo and in vitro characterization of nine of the “tight-binding” repressors indicates that: (1) they cause 1.5- to 6-fold decreases in basal β-galactosidase specific activities relative to the parental Q wild-type repressor, and have up to 30-fold increases in operator affinity in vitro. (2) With a few exceptions, the tight-binding repressors show the same relative decreases in basal β-galactosidase specific activities for a wide range of operator types (o⁺ and o^c). (3) With two exceptions, the tight-binding repressors show normal or nearly normal affinities for the inducer, isopropyl-β-d-thiogalactoside, although the concentrations of inducer needed to release various repressors from the o⁺ operator vary greatly.