DNA binding characteristics of lactose repressor and the trypsin-resistant core repressor

The nonspecific DNA binding capacity of repressor protein has been assessed by boundary sedimentation of repressor and calf thymus DNA fragmented by shearing and by nitrocellulose ultrafiltration employing labeled lambdaplac DNA in the presence of inducer concentrations sufficient to insure dissociation of repressor from the operator region of the DNA. These methods gave values in good agreement with values previously reported in the literature. The association constants for the interaction of repressor with operator DNA fragments and lambdaplac DNA have been measured and found to differ by approximately 100-fold at low salt concentrations, but the difference decreases to 4-fold at salt concentrations near the physiological value. The equilibrium association constant for the repressor-operator DNA fragment is significantly less sensitive to salt concentration than the corresponding constant for lambdaplac DNA. Inducer decreases the salt concentration dependence of repressor-operator DNA fragment only slightly. Measurement of the association constants for the interaction of the trypsin-resistant core protein with operator DNA fragment and lambdaplac DNA indicate that the core protein binds to the two DNA's with the same affinity. This result contrasts with the differential affinity of intact repressor for these two DNA's. In addition, the core protein association constants for operator DNA fragment and lambdaplac DNA display minimal dependence on the salt concentration. These results suggest a role for nonionic interactions in the binding of core protein to operator DNA.