A procedure has been devised that allows selection of mutants defective in the 3-methylgalactoside transport system (mgl) of Escherichia coli. This procedure utilizes the compound 2R-glyceryl-13-Dgalactopyranoside (glycerylgalactoside), which is known to be transported by only two transport system in E. coli, namely, the lactose and the W-methylgalactoside transport systems. Mutants lacking glycerol-3-phosphate dehydrogenase (glpD) are sensitive to glycerol. Similarly, mutants lacking uridine

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... phosphate-galactose-4-epimerase (galE) are sensitive to galactose. Glycerylgalactoside is an inducer of the lactose operon and also a substrate for f-galactosidase. Thus, a mgl+ glpD galE lacY strain will not grow in the presence of glycerylgalactoside owing to accumulated glycerol-3-phosphate, galactose-1-phosphate, and uridine diphosphate-galactose. We have constructed such a strain and shown that mgl mutants can be obtained by selecting for those that grow in the presence of glycerylgalactoside.