Mutants of Citrobacter freundii That Transport and Utilize Melibiose

We have isolated mutants of Citrobacter freundii that can grow on melibiose. Inducible α-galactosidase activity and melibiose transport activity were detected in the mutant cells but not in the wild-type cells. We detected a DNA region which hybridized with melB (the gene for the melibiose transporter) DNA of Escherichia coli in the chromosomal DNA of wild-type C. freundii. Protons, but not sodium ions, were found to be the coupling cations for melibiose (and methyl-β-d-thiogalactoside) transport in the mutant cells.