Plasmids, loss of lactose metabolism, and appearance of partial and full lactose-fermenting revertants in Streptococcus cremoris B1

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Abstract
The unstable ability to metabolize lactose (lac) via the phosphoenolpyruvate-phosphotransferase system (PTS) was examined in S. cremoris B1. The presence of functional lactose-specific PTS enzymes was correlated with the presence of a distinct plasmid species. Characterization of DNA extracted from lactose-positive (Lac+) S. cremoris B1 revealed 2 plasmids having MW of 9 .times. 106 and 36 .times. 106. An acriflavine (BC1)-induced, lactose-negative (Lac-) mutant possessed no plasmids and was devoid of all 3 lac-specific PTS enzymes. A Lac- mutant (DA2) isolated by growing at elevated temperatures only possessed the 9 .times. 106-dalton plasmid and also lacked the lac PTS enzymes. A spontaneous Lac- mutant possessed the 9 .times. 106- and 36 .times. 106-dalton plasmids. This mutant displayed FIII-lac and phospho-.beta.-D-galactosidase (P-.beta.-gal) activity but was deficient in EII-lac activity. The spontaneous Lac- strain reverted to full and partial lactose-fermenting phenotypes having FIII-lac, EII-lac and P-.beta.-gal activities. BC1 and DA2 Lac- mutants reverted only to the partial lactose-fermenting phenotype having P-.beta.-gal activity; EII-lac and FIII-lac activities were absent. The genetic determinants for EII-lac, FIII-lac and P-.beta.-gal are apparently located on the 36 .times. 106-dalton plasmid in S. cremoris B1. Evidence for a 2nd chromosomally associated P-.beta.-gal gene operating in the partial lactose-fermenting revertants is presented.