REVERSIBLE ASSOCIATION OF THE EQUILIBRIUM UNFOLDING INTERMEDIATE OF LAMBDA-CRO REPRESSOR

An extended differentiated scanning calorimetry study of the wild-type Cro repressor and of its V55C mutant has revealed a significant conce ntration dependence of the melting profiles, even though the two polyp eptide chains forming the active repressor molecule are covalently bou nd within the mutant. An analysis of the temperature dependencies of t he partial molar heat capacity suggests that in both cases equilibrium unfolding occurs via a highly-populated intermediate state correspond ing to polypeptide tetramers. The results of thermodynamic analysis ar e confirmed by direct glutaraldehyde cross-linking experiments. Judgin g by heat effects and circular dichroism data, this intermediate state regains about 50% of the ordered structure and melts co-operatively. (C) 1996 Academic Press Limited.