SUMMARY: Mutants of K12 defective in the gene () no longer utilize glucose or -acetylglucosamine in preference to lactose, but competition between either of these sugars and another that also enters by a phosphotransferase (PT) mechanism is not affected. In this they differ from () mutants. In mutants, glucose does not exclude any other sugar, though -acetylglucosamine still does so. In mutants that are also the phosphoenolpyruvate-dependent phosphorylation of glucose or methyl α-glucoside is reduced by 90%: -acetylglucosamine phosphorylation is not affected. The mutation does not affect the phosphorylation of either of these compounds. The wild-type alleles and are dominant in λ heterozygotes. Glucose inhibits the lactose permease of wild-type cells, but only when the permease is present in low amounts. The inhibition is also relieved (1) by induction of another transport system that is subject to regulation by the system or (2) by an mutation. We suggest that the gene specifies a protein that, in cells transporting certain sugars by a PT mechanism, acts to inhibit active transport systems. The protein is present in limiting concentration in the cell, sufficient only to inhibit the basal, uninduced, level of the active transport systems. In consequence the inducer (or its precursor) may be excluded from the cell and induction thus prevented.


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