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

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Abstract
Development of strains of dairy streptococci that improve or control rate of lactic acid production would be of value in dairy fermentations. Improvement of the ability of dairy starter cultures to metabolize lactose has been limited because of inability to examine and characterize these plasmid-linked genes. A study was initiated to define further the genetic and enzymatic basis of lactose utilization of S. lactis C2. Lactose-negative mutants were isolated by UV irradiation or ethyl methanesulfonate mutagenesis from a C2 strain in which genes for lactose utilization became integrated into the chromosome. Mutants were examined for phospho-.beta.-galactoside activity and for their ability to accumulate 14C thiomethyl-.beta.-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 were obtained. Lactose-negative mutants missing only the lactose-specific Enzyme IIlac (EIIlac) or Factor IIIlac (FIIlac) were identified. A mutant that retained the lactose-negative phenotype, even though it possessed EIIlac, FIIIlac and phospho-.beta.-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 3 types.