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

The wavelength dependence of the intrinsic tryptophan fluorescence lifetime of a series of mutants of the trp repressor protein was characterized in b oth the native and the denatured states. These mutants belong to a particul ar class, called superrepressors, as their phenotype, when expressed in viv o, is to repress transcription at lower concentrations of the corepressor, tryptophan. II has been demonstrated previously that these mutations result in distinct and profound modifications of the structural and dynamic: prop erties of the protein [Reedstrom and Royer (1995) J. Mel. Biol. 253, 266; R eedstrom et al. (1996) J. Mel. Biol. 263, 32; Smith et nl. (1995) Biochemis try 34, 13183]. The present observations reveal that in the native state, t hese 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.