Abstract
The nicotinic acid synthesis is used in both secondary and primary metabolisms. A part of nicotine is derived from nicotinic acid in tobacco, whereas the primary metabolite NAD is also synthesized from nicotinic acid. In spite of the importance of this metabolism, the biosynthetic pathway has not been established in higher plants. We found expression level of quinolinate synthetase (QS)-like gene was lower in low-nicotine tabacco mutant nic1nic2 than that in wild type. cDNA homologues to QS and L-aspartate oxidase (AO), which are both involved in the nicotinic acid synthesis in bacteria, were obtained from RIKEN Arabidopsis cDNA library and were found to complement growth of E. coli mutants deficient in QS or AO genes. Our results suggest that in higher plants nicotinic acid is produced from L-aspartate, as in bacteria. This pathway is different from the one starting from L-tryptophan in mammals and fungi.
