Dimer-dimer interfaces of the λ-repressor are different in liganded and free states

Abstract

λ-Repressor dimers associate in solution to form tetramers and higher order structures. Dimer-dimer contact is also crucial in cooperative binding to adjacent operators. Fluorescence quenching studies indicate that the tryptophan 230 environment is significantly different in unliganded and adjacent operator-bound tetramers. Acrylodan attached to Cys 235, in a mutant F235C repressor, is also in different environments in the unliganded and adjacent operator bound tetramers. Thermodynamics of protein association, measured by fluorescence anisotropy, indicate that, whereas free repressor dimer association is strongly enthalpy driven, the single-operator (O_R1)-bound repressor dimer association is largely entropy driven with little change in enthalpy. Single-operator-bound dimer association to the corresponding tetramer does not lead to any significant change in tryptophan 230 environment, as was seen in the case of the free repressor. Data are also presented to support the contention that, under the conditions of this study, the free repressor association is predominantly from dimer to tetramer and then to octamer, unlike the dimer to octamer transition observed under a different condition. The results presented here point toward the conclusion that the λ-repressor dimer-dimer interface is significantly different in the free and the operator-bound states and that operator binding plays a crucial role in changing the nature of the dimer-dimer interface.