Further inducibility of a constitutive system: ultrainduction of the gal operon.
In wild-type Escherichia coli, expression of the gal operon is negatively regulated by the Gal repressor and is induced 10- to 15-fold when the repressor is inactivated by an inducer. In strains completely deleted for galR, the gene which encodes the Gal repressor, the operon is derepressed by only 10-fold without an inducer. But this derepression is increased further by threefold during cell growth in the presence of an inducer, D-galactose or D-fucose. This phenomenon of extreme induction in the absence of Gal repressor is termed ultrainduction--a manifestation of further inducibility in a constitutive setup. Construction and characterization of gene and operon fusion strains between galE and lacZ, encoding beta-galactosidase as a reporter gene, show that ultrainduction occurs at the level of transcription and not translation. Transcription of the operon, from both the cyclic AMP-dependent P1 and the cyclic nucleotide-independent P2 promoters, is subject to ultrainduction. The wild-type galR+ gene has an epistatic effect on ultrainducibility: ultrainduction is observed only in cells devoid of Gal repressor protein. Titration experiments show the existence of an ultrainducibility factor that acts like a repressor and functions by binding to DNA segments (operators) to which Gal repressor also binds to repress the operon.
Tokeson
J P
JP
Laboratory of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892.
Garges
S
S
Adhya
S
S
D004252:DNA Mutational Analysis; D004790:Enzyme Induction; D004843:Epistasis, Genetic; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D005686:Galactokinase / Q000235:genetics; D005690:Galactose; D015964:Gene Expression Regulation, Bacterial; D009875:Operator Regions, Genetic; D009876:Operon; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D012097:Repressor Proteins / Q000502:physiology*; D014158:Transcription, Genetic
D029968:Escherichia coli Proteins; C050993:Galactose repressor proteins; D012097:Repressor Proteins; D005686:Galactokinase; D005690:Galactose