Multiphasic denaturation of the λ repressor by urea and its implications for the repressor structure

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Abstract
Urea denaturation of the λ repressor has been studied by fluorescence and circular dichroic spectroscopies. Three phases of denaturation could be detected which we have assigned to part of the C‐terminal domain, N‐terminal domain and subunit dissociation coupled with further denaturation of the rest of the C‐terminal domain at increasing urea concentrations. Acrylamide quenching suggests that at least one of the three tryptophan residues of the λ repressor is in a different environment and its emission maximum is considerably blue‐shifted. The transition in low urea concentration (midpoint approximately 2 M) affects the environment of this tryptophan residue, which is located in the C‐terminal domain. Removal of the hinge and the N‐terminal domain shifts this transition towards even lower urea concentrations, indicating the presence of interaction between hinge on N‐terminal and C‐terminal domains in the intact repressor.