title: New function for Escherichia coli xanthosine phophorylase (xapA): genetic and biochemical evidences on its participation in NAD(+) salvage from nicotinamide.
abstract: 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.
authors: Wei-Ren
authors: WR
authors: Dong
authors: Cen-Cen
authors: CC
authors: Sun
authors: College of Life Sciences, Zhejiang University, Hangzhou 310058, People's Republic of China. gzhu@cvm.tamu.edu.
authors: Guan
authors: G
authors: Zhu
authors: Shi-Hua
authors: SH
authors: Hu
authors: Li-Xin
authors: LX
authors: Xiang
authors: Jian-Zhong
authors: JZ
authors: Shao
mesh_terms: D003470:Culture Media / Q000737:chemistry; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism*; D055786:Gene Knockout Techniques; D009243:NAD / Q000378:metabolism*; D009536:Niacinamide / Q000378:metabolism*; D010430:Pentosyltransferases / Q000378:metabolism*; D011683:Purine-Nucleoside Phosphorylase / Q000378:metabolism*
chemical_list: D003470:Culture Media; D029968:Escherichia coli Proteins; D009243:NAD; D009536:Niacinamide; D010430:Pentosyltransferases; C000591543:xanthosine phophorylase, E coli; C028449:xanthosine phosphorylase; D011683:Purine-Nucleoside Phosphorylase