Xanthosine utilization in Salmonella enterica serovar Typhimurium is recovered by a single aspartate-to-glycine substitution in xanthosine phosphorylase.

xapABR from Salmonella enterica was analyzed and compared with the corresponding Escherichia coli genes. xapB and xapR, but not xapA, encode functional proteins. An S. enterica XapA(Asp72Gly) mutant that restores the phosphorolytic activity was selected. The purified mutant enzyme has different kinetic constants than the E. coli enzyme but similar substrate specificity.