摘要 : We isolated and characterized mutant repressors (GalR) of the gal operon in Escherichia coli. These repressors (super-repressors), called GalR(s), have a non-inducible phenotype. Repression of the gal operon by super-repressors ca... 展开 We isolated and characterized mutant repressors (GalR) of the gal operon in Escherichia coli. These repressors (super-repressors), called GalR(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 of them, galR(uv7)(s) and galR(78)(s), were located in the proposed sugar binding domains of the repressor. The repressor from wild-type (galR(+)), as well as from mutant galR(uv7)(s), was purified and characterized biochemically. The results showed that, like wild-type GalR(+), GalR(uv7)(s) 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(+), GalR(uv7)(s) is much less sensitive to the presence of the inducer, D-galactose. The affinity of D-galactose to GalR(uv7)(s) 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 GalR(uv7)(s) 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. (C) 1995 Academic Press Limited [References: 41] 收起