Identification of a gene in Staphylococcus xylosus encoding a novel glucose uptake protein

By transposon Tn917 mutagenesis, two mutants of Staphylococcus xylosus were isolated that showed higher levels of beta-galactosidase activity in the p resence of glucose than the wild type. Both transposons integrated in a gen e, designated glcU, encoding a protein involved in glucose uptake in S. xyl osus, which is followed by a glucose dehydrogenase gene (gdh). Glucose-medi ated repression of beta-galactosidase, alpha-glucosidase, and beta-glucuron idase activities was partially relieved in the mutant strains, while repres sion by sucrose or fructose remained as strong as in the wild type. In addi tion to the pleiotropic regulatory effect, integration of the transposons i nto glcU reduced glucose dehydrogenase activity, suggesting cotranscription of glcU and gdh. Insertional inactivation of the gdh gene and deletion of the glcU gene without affecting gdh expression showed that loss of GlcU fun ction is exclusively responsible for the regulatory defect. Reduced glucose repression is most likely the consequence of impaired glucose uptake in th e glcU mutant strains. With cloned glcU, an Escherichia coli mutant deficie nt in glucose transport could grow with glucose as sole carbon source, prov ided a functional glucose kinase was present. Therefore, glucose is interna lized by glcU in nonphosphorylated form. A gene from Bacillus subtilis, ycx E, that is homologous to glcU, could substitute for glcU in the E. coli glu cose growth experiments and restored glucose repression in the S, xylosus g lcU mutants. Three more proteins dth high levels of similarity to GlcU and YcxE are currently in the databases. It appears that these proteins constit ute a novel family whose members are involved in bacterial transport proces ses. GlcU and YcxE are the first examples whose specificity, glucose, has b een determined.