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Selection Procedure for Mutants Defective in the ß-Methylgalactoside Transport System of Escherichia coli Utilizing the Compound 2R-Glyceryl-ß-D-Galactopyranoside

¹ Biochemical Research Laboratory, Massachusetts General Hospital and the Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts 02114

ABSTRACT

A procedure has been devised that allows selection of mutants defective in the ß-methylgalactoside transport system (mgl) of Escherichia coli. This procedure utilizes the compound 2R-glyceryl-ß-D-galactopyranoside (glycerylgalactoside), which is known to be transported by only two transport system in E. coli, namely, the lactose and the ß-methylgalactoside transport systems. Mutants lacking glycerol-3-phosphate dehydrogenase (glpD) are sensitive to glycerol. Similarly, mutants lacking uridine diphosphate-galactose-4-epimerase (galE) are sensitive to galactose. Glycerylgalactoside is an inducer of the lactose operon and also a substrate for ß-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.