Characterization of Lactose-Fermenting Revertants from Lactose-Negative Streptococcus lactis C2 Mutants1

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Partial lactose-fermenting revertants from lactose-negative (lac−) mutants of Streptococcus lactis C2 appeared on a lawn of lac− cells after 3 to 5 days of incubation at 25 C. The revertants grew slowly on lactose with a growth response similar to that for cryptic cells. In contrast to lac+S. lactis C2, the revertants were defective in the accumulation of [14C]thiomethyl-β-d-galactoside, indicating that they were devoid of a transport system. Hydrolysis of o-nitrophenyl-β-d-galactoside-6-phosphate by toluene-treated cells confirmed the presence of phospho-β-d-galactosidase (P-β-gal) in the revertant. However, this enzyme was induced only when the cells were grown in the presence of lactose; galactose was not an inducer. In lac+S. lactis C2, enzyme induction occurred in lactose- or galactose-grown cells. The revertants were defective in EII-lactose and FIII-lactose of the phosphoenolpyruvate-dependent phosphotransferase system. Galactokinase activity was detected in cell extracts of lac+S. lactis C2, but the activity was 9 to 13 times higher in extracts from the revertant and lac−, respectively. This suggested that the lac− and the revertants use the Leloir pathway for galactose metabolism and that galactose-1-phosphate rather than galactose-6-phosphate was being formed. This may explain why lactose, but not galactose, induced P-β-gal in the revertants. Because the revertant was unable to form galactose-6-phosphate, induction could not occur. This compound would be formed on hydrolysis of lactose phosphate. The data also indicate that galactose-6-phosphate may serve not only as an inducer of the lactose genes in S. lactis C2, but also as a repressor of the Leloir pathway for galactose metabolism.

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