DsdX is the second D-serine transporter in uropathogenic Escherichia coli clinical isolate CFT073

D-Serine is an amino acid present in mammalian urine that is inhibitory to Escherichia coli strains lacking a functional dsdA gene. Counterintuitively, a dsd4 strain of E. coli clinical isolate CFT073 hypercolonizes the bladder and kidneys of mice relative to wild type during a coinfection in the murine model of urinary tract infection. We are interested in the mechanisms for uptake Of D-serine in CFT073. D-Serine enters E. coli K-12 via CycA, the D-alanine transporter and D-cycloserine sensitivity locus. CFT073 cycA can grow on minimal medium with D-serine as a sole carbon source. The dsdX gene of the dsdCXA locus is a likely candidate for an additional D-serine transporter based on its predicted amino acid sequence similarity to gluconate transporters. In minimal medium, CFT073 dsdX can grow on D-serine as a sole carbon source; however, CFT7073 dsdX cycA cannot. Additionally, CFT073 dsdXA cycA is not sensitive to inhibitory concentrations Of D-serine during growth on glycerol and D-serine minimal medium. D-[C-14] serine uptake experiments with CFT073 dsdX cycA harboring dsdX or cycA recombinant plasmids confirm that D-serine is able to enter E. coli cells via CycA or DsdX. In whole-Cell D- [C-14] serine uptake experiments, DsdX has an apparent K-m of 58.75 mu M and a V-max Of 75.96 nmol/min/mg, and CycA has an apparent K-m of 82.40 mu M and a V-max of 58.90 nmol/min/mg. Only D-threonine marginally inhibits DsdX-mediated D-serine transport, whereas D-alanine, glycine, and D-Cycloserine inhibit CycA-mediated D-serine transport. DsdX or CycA is sufficient to transport physiological quantities of D-serine, but DsdX is a D-serine-specific permease.