title: Influence of the lactose plasmid on the metabolism of galactose by Streptococcus lactis.
abstract: Streptococcus lactis strain DR1251 was capable of growth on lactose and galactose with generation times, at 30 degrees 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 K(m) of 5 x 10(-5) M for lactose and 2 x 10(-2) M for galactose. beta-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 degrees C with glucose as the growth substrate. Loss of ability to metabolize lactose was accompanied by the loss of a 32-megadalton plasmid, pDR(1), 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 beta-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.
mesh_terms: D005285:Fermentation; D005686:Galactokinase / Q000378:metabolism; D005690:Galactose / Q000378:metabolism*; D013294:Lactococcus lactis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D007785:Lactose / Q000378:metabolism; D007786:Lactose Factors; D010770:Phosphotransferases / Q000378:metabolism; D010957:Plasmids; D013696:Temperature; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D010770:Phosphotransferases; D005686:Galactokinase; D001616:beta-Galactosidase; D007785:Lactose; D005690:Galactose