Determination of the ligand-binding characteristics of several tight-binding mutants of the lactose repressor protein.
Several tight-binding mutants of the lactose repressor protein have been characterized with respect to their fluorescence properties and their inducer, operator and nonspecific DNA-binding constants. The tryptophan fluorescence emission spectra for the mutants and the wild-type repressor are quite similar. However, alterations in the Stern-Volmer constants for iodide quenching of the tryptophans in the mutant proteins compared to wild-type suggest differences in the local environment or solvent accessibility for these amino acids in the tight-binding repressors. The inducer-binding affinities and association rate constants of the mutant proteins and protein-operator DNA fragment complexes are also altered compared to wild-type. The extents of these changes vary among the different mutant repressors. The nonspecific DNA-binding affinities of the mutant proteins are 2--3-fold greater than the wild-type repressor, and the affinities of the tight-binding proteins for a 29 base-pair operator DNA fragment are also increased, though to a varying extent depending upon the mutant. The phenotypic behavior of these proteins in vivo can be partially explained by these results obtained in vitro; however, it is likely that there are additional factors responsible for the tight-binding behavior of the proteins that were not detectable in these experiments.
D000483:Alleles; D004269:DNA, Bacterial / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics*; D007700:Kinetics; D009154:Mutation; D011485:Protein Binding; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D013045:Species Specificity; D013050:Spectrometry, Fluorescence; D014157:Transcription Factors / Q000378:metabolism*
D004269:DNA, Bacterial; D012097:Repressor Proteins; D014157:Transcription Factors