title: Characterization of lactose-fermenting revertants from lactose-negative Streptococcus lactis C2 mutants.
abstract: 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 [(14)C]thiomethyl-beta-d-galactoside, indicating that they were devoid of a transport system. Hydrolysis of o-nitrophenyl-beta-d-galactoside-6-phosphate by toluene-treated cells confirmed the presence of phospho-beta-d-galactosidase (P-beta-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-beta-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.
mesh_terms: D002250:Carbon Radioisotopes; D004790:Enzyme Induction; D005285:Fermentation; D005690:Galactose / Q000378:metabolism; D005696:Galactosidases / Q000378:metabolism; D005809:Genes, Regulator; D005816:Genetic Complementation Test; D005947:Glucose / Q000378:metabolism; D006027:Glycosides / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D013294:Lactococcus lactis / Q000201:enzymology / Q000378:metabolism*; D007785:Lactose / Q000378:metabolism*; D009154:Mutation; D009578:Nitrobenzenes; D010770:Phosphotransferases / Q000378:metabolism; D013865:Thioglycosides / Q000378:metabolism; D014050:Toluene / Q000494:pharmacology; D014161:Transduction, Genetic
chemical_list: D002250:Carbon Radioisotopes; D006027:Glycosides; D009578:Nitrobenzenes; D013865:Thioglycosides; D014050:Toluene; D010770:Phosphotransferases; D005696:Galactosidases; D005947:Glucose; D007785:Lactose; D005690:Galactose