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


  • 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.

published proceedings

  • BMC Microbiol

author list (cited authors)

  • Dong, W., Sun, C., Zhu, G., Hu, S., Xiang, L., & Shao, J.

citation count

  • 18

complete list of authors

  • Dong, Wei-Ren||Sun, Cen-Cen||Zhu, Guan||Hu, Shi-Hua||Xiang, Li-Xin||Shao, Jian-Zhong

publication date

  • January 2014