Galactose Repression of β-Galactosidase Induction in Escherichia coli

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Abstract
Beggs, William H. (University of Minnesota, Minneapolis), and Palmer Rogers. Galactose repression of β-galactosidase induction in Escherichia coli. J. Bacteriol. 91:1869–1874. 1966.—Galactose repression of β-galactosidase induction in Escherichia coli was investigated to determine whether the galactose molecule itself is the catabolite repressor of this enzyme system. Without exception, β-galactosidase induction by cells grown in a synthetic salts medium with lactate or glycerol as the carbon source was more strongly repressed by glucose than by galactose. This relationship existed even when the organism was previously grown in the synthetic medium containing galactose as the source of carbon. Two observations suggested that the ability of galactose to repress β-galactosidase formation by Escherichia coli depends directly upon the cells9 capacity to catabolize galactose. First, galactose repression of β-galactosidase synthesis was markedly enhanced in bacteria tested subsequent to gratuitous induction of the galactose-degrading enzymes with d-fucose. Second, galactose failed to exert a repressive effect on β-galactosidase in a galactose-negative mutant lacking the first two enzymes involved in galactose catabolism. Glucose completely repressed enzyme formation in this mutant. This same mutant, into which the genes for inducible galactose utilization had been introduced previously by transduction, again exhibited galactose repression. Pyruvate was found to be at least as effective as galactose in repressing β-galactosidase induction by cells grown in synthetic salts medium plus glycerol. It is concluded that the galactose molecule itself is not the catabolite repressor of β-galactosidase, but that repression is exerted through some intermediate in galactose catabolism.