THE NONINDUCIBLE NATURE OF SUPER-REPRESSORS OF THE GAL OPERON IN ESCHERICHIA-COLI

We isolated and characterized mutant repressors (GalR) of the gal oper on in Escherichia coli. These repressors (super-repressors), called Ga lR(s), have a non-inducible phenotype. Repression of the gal operon by super-repressors cannot be lifted by inducer. The mutant galR genes, galR(s), have been cloned and the mutational changes determined. Two o f them, galR(uv7)(s) and galR(78)(s), were located in the proposed sug ar binding domains of the repressor. The repressor from wild-type (gal R(+)), as well as from mutant galR(uv7)(s), was purified and character ized biochemically. The results showed that, like wild-type GalR(+), G alR(uv7)(s) binds to DNA normally and represses transcription from the P1 promoter and stimulates that from the P2 promoter of the gal opero n. Nevertheless, compared to GalR(+), GalR(uv7)(s) is much less sensit ive to the presence of the inducer, D-galactose. The affinity of D-gal actose to GalR(uv7)(s) is 10 to 30-fold lower, as measured by the effe ct 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 GalR(uv7)(s) is because of a defect in D-galactose binding rather than a defect in the ligand-induced allo steric change or increased affinity for the operator. (C) 1995 Academi c Press Limited