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

Abstract

A procedure has been devised that allows selection of mutants defective in the β-methylgalactoside transport system ( mgl ) of Escherichia coli . This procedure utilizes the compound 2R-glyceryl-β- d -galactopyranoside (glycerylgalactoside), which is known to be transported by only two transport system in E. coli , namely, the lactose and the β-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 β-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.