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Background  In an effort to reconstitute the NAD +  synthetic pathway in  Escherichia coli  ( E. coli ), we produced a set of gene knockout mutants with deficiencies in previously well-defined NAD +  de novo  and salvage pathways. Unexpectedly, the mutant deficient in NAD +  de novo  and salvage pathway  I  could grow in M9/nicotinamide medium, which was contradictory to the proposed classic NAD +  metabolism of  E. coli . Such  E. coli  mutagenesis assay suggested the presence of an undefined machinery to feed nicotinamide into the NAD +  biosynthesis. We wanted to verify whether xanthosine phophorylase (xapA) contributed to a new NAD +  salvage pathway from nicotinamide.    Results  Additional knockout of  xapA  further slowed down the bacterial growth in M9/nicotinamide medium, whereas the complementation of  xapA  restored the growth phenotype. To further validate the new function of xapA, we cloned and expressed  E. coli  xapA as a recombinant soluble protein. Biochemical assay confirmed that xapA was capable of using nicotinamide as a substrate for nicotinamide riboside formation.    Conclusions  Both the genetic and biochemical evidences indicated that xapA could convert nicotinamide to nicotinamide riboside in  E. coli , albeit with relatively weak activity, indicating that xapA may contribute to a second NAD +  salvage pathway from nicotinamide. We speculate that this xapA-mediated NAD +  salvage pathway might be significant in some bacteria lacking NAD +  de novo  and NAD +  salvage pathway  I  or  II , to not only use nicotinamide riboside, but also nicotinamide as precursors to synthesize NAD + . However, this speculation needs to be experimentally tested.

bmc microbiology

New function for Escherichia coli xanthosine phophorylase (xapA): genetic and biochemical evidences on its participation in NAD+ salvage from nicotinamide