Influence of the Lactose Plasmid on the Metabolism of Galactose by Streptococcus lactis


Streptococcus lactis strain DR1251 was capable of growth on lactose and galactose with generation times, at 30°C, of 42 and 52 min, respectively. Phosphoenolpyruvate-dependent phosphotransferase activity for lactose and galactose was induced during growth on either substrate. This activity had an apparent Km of 5 × 10−5 M for lactose and 2 × 10−2 M for galactose. β-d-Phosphogalactoside galactohydrolase activity was synthesized constitutively by these cells. Strain DR1251 lost the ability to grow on lactose at a high frequency when incubated at 37°C with glucose as the growth substrate. Loss of ability to metabolize lactose was accompanied by the loss of a 32-megadalton plasmid, pDR1, and Lac− isolates did not revert to a Lac+ phenotype. Lac− strains were able to grow on galactose but with a longer generation time. Galactose-grown Lac− strains were deficient in β-d-phosphogalactoside galactohydrolase activity and phosphoenolpyruvate phosphotransferase activity for both lactose and galactose. There was also a shift from a predominantly homolactic to a heterolactic fermentation and a fivefold increase in galactokinase activity, relative to the Lac+ parent strain grown on galactose. These results suggest that S. lactis strain DR1251 metabolizes galactose primarily via the tagatose-6-phosphate pathway, using a lactose phosphoenolpyruvate phosphotransferase activity to transport this substrate into the cell. Lac− derivatives of strain DR1251, deficient in the lactose phosphoenolpyruvate phosphotransferase activity, appeared to utilize galactose via the Leloir pathway.

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