Probing the relation between protein structure and intrinsic tryptophan fluorescence using superrepressor mutants of thetrp repressor

The wavelength dependence of the intrinsic tryptophan fluorescence lifetime of a series of mutants of thetrp repressor protein was characterized in both the native and the denatured states. These mutants belong to a particular class, called superrepressors, as their phenotype, when expressedin vivo, is to repress transcription at lower concentrations of the corepressor, tryptophan. It has been demonstrated previously that these mutations result in distinct and profound modifications of the structural and dynamic properties of the protein [Reedstrom and Royer (1995)J. Mol. Biol.253, 266; Reedstromet al. (1996)J. Mol. Biol.264, 32; Smithet al. (1995)Biochemistry34, 13183]. The present observations reveal that in the native state, these structural and dynamic modifications result in subtle, yet significant alterations in the intrinsic tryptophan fluorescence decay characteristics. Surprisingly, significant differences in the fluorescence decays between the mutants and the wild-type protein were also observed for the guanidine hydrochloride unfolded states. These results are discussed in terms of the various models which have been proposed to explain the decay properties of tryptophan in proteins.