Saccharomyces cerevisiae QNS1 codes for NAD + synthetase that is functionally conserved in mammals

NAD + , an essential molecule involved in a variety of cellular processes, is synthesized through de novo and salvage pathways. NAD + synthetase catalyses the final step in both pathways. Here we show that this enzyme is encoded by the QNS1 gene in Saccharomyces cerevisiae . Expression of Escherichia coli or Bacillus subtilis NAD + synthetases was able to suppress the lethality of a qns1 deletion, while a B. subtilis NAD + synthetase mutant with lowered catalytic activity was not. Overexpression of QNS1 tagged with HA led to elevated levels of NAD + synthetase activity in yeast extracts, and this activity can be recovered by immunoprecipitation using anti‐HA antibody. An allele of QNS1 was constructed that carries a point mutation predicted to reduce the catalytic activity. Overexpression of this allele, qns1 G521E , failed to elevate NAD + synthetase levels and qns1 G521E could not rescue the lethality caused by the depletion of Qns1p. These results demonstrate that NAD + synthetase activity is essential for cell viability. A GFP‐tagged version of Qns1p displayed a diffuse localization in both the nucleus and the cytosol. Finally, the rat homologue of QNS1 was cloned and shown to functionally replace yeast QNS1 , indicating that NAD + synthetase is functionally conserved from bacteria to yeast and mammals. Copyright © 2003 John Wiley & Sons, Ltd.