Functional and biochemical characterization of Escherichia coli sugar efflux transporters

A family of bacterial transporters, the SET (sugar efflux transporter) fami ly, has been recently reported (Liu, J. Y., Miller, P. F., Gosink, M., and Olson, E. R. (1999) Mol. Microbiol. 31, 1845-1851), In this study, the bioc hemical and cell biological properties of the three Escherichia coil member s (SetA, SetB, and SetC) of the family are characterized. We show that both SetA and SetB can transport lactose and glucose. In addition, SetA has bro ad substrate specificity, with preferences for glucosides or galactosides w ith alkyl or aryl substituents, Consistent with the observed in vitro subst rate specificities, strains that hyperexpress SetA or SetB are desensitized to lactose analogues as measured by induction of the lac operon, In additi on, strains that hyperexpress SetA are resistant to the growth inhibitory s ugar analogue o-nitrophenyl-beta-D-thiogalactoside. Strains disrupted for a ny one or all of the set genes are viable and show no defects in lactose ut ilization nor increased sensitivity to inducers of the lac operon and nonme tabolizable sugar analogues. The data suggest that the set genes are either poorly expressed under normal laboratory growth conditions or are redundan t with other cellular gene products.