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Isolation and Partial Characterization of Lactose-Negative Mutants from Streptococcus lactis C2 Defective in the Phosphotransferase System¹

Department of Food Science and Nutrition, University of Minnesota, 1334 Eckles Avenue, St. Paul 55108

ABSTRACT

Development of strains of dairy streptococci that improve or control rate of lactic acid production would be of value in dairy fermentations. However, improvement of the ability of dairy starter cultures to metabolize lactose has been limited because of inability to examine and characterize these plasmidlinked genes. A study was initiated to define further the genetic and enzymatic basis of lactose utilization of Streptococcus lactis C2. Lactose-negative mutants were isolated by ultraviolet irradiation or ethyl methanesulfonate mutagenesis from a C2 strain in which genes for lactose utilization had become integrated into the chromosome. Mutants were examined for phospho-ß-galactosidase activity and for their ability to accumulate [carbon-14] thiomethyl-ß-D-galactoside. Selected mutants were tested for enzyme complementation by Staphylococcus aureus mutants with known specific defects in lactose-utilizing enzymes. Genetic complementation via transduction between lactose-negative derivatives to yield lactose-positive recombinants also was obtained. Lactose-negative mutants missing only the lactose-specific Enzyme II^lac (EII^lac) or Factor III^lac (FIII^lac) were identified. A mutant that retained the lactose-negative phenotype, even though it possessed EII^lac, FIII^lac, and phospho-ß-galactosidase activities, was isolated. Single site mutations in these lactose-negative variants were supported by the observation that full lactose-fermenting revertants were obtainable from each of the three types.

¹ Paper No. 13523 of the Scientific Journal Series of the Minnesota Agricultural Experiment Station. This research was conducted under Minnesota Agricultural Experiment Station No. 18—62.