title: The non-inducible nature of super-repressors of the gal operon in Escherichia coli.
abstract: We isolated and characterized mutant repressors (GalR) of the gal operon in Escherichia coli. These repressors (super-repressors), called GalRs, have a non-inducible phenotype. Repression of the gal operon by super-repressors cannot be lifted by inducer. The mutant galR genes, galRs, have been cloned and the mutational changes determined. Two of them, galRuv7s and galR78s, were located in the proposed sugar binding domains of the repressor. The repressor from wild-type (galR+), as well as from mutant galRuv7s, was purified and characterized biochemically. The results showed that, like wild-type GalR+, GalRuv7s binds to DNA normally and represses transcription from the P1 promoter and stimulates that from the P2 promoter of the gal operon. Nevertheless, compared to GalR+, GalRuv7s is much less sensitive to the presence of the inducer, D-galactose. The affinity of D-galactose to GalRuv7s is 10 to 30-fold lower, as measured by the effect of the inducer on GalR tryptophan fluorescence; GalR complexes with DNA and on GalR repression of transcription. Our results suggest that the super-repressor phenotype of GalRuv7s is because of a defect in D-galactose binding rather than a defect in the ligand-induced allosteric change or increased affinity for the operator.
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins; D005690:Galactose / Q000235:genetics* / Q000378:metabolism; D005798:Genes, Bacterial; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D009876:Operon; D010641:Phenotype; D011401:Promoter Regions, Genetic; D012097:Repressor Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D017931:DNA Primers; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; C050993:Galactose repressor proteins; D012097:Repressor Proteins; D005690:Galactose