Abstract
Quinolinic acid is a key intermediate in biosynthesis of nicotinamide adenine dinucleotide (NAD). Mammals and fungi produce quinolinic acid from L-tryptophan, whereas E. coli synthesizes it from L-aspartate. In the Arabidopsis genome, putative genes for enzymes involved in the E. coli-type pathway, L-aspartate oxidase (AO) and quinolinate synthetase (QS), are present, but those in the mammal-type pathway are absent. AO and QS genes from Arabidopsis complemented growth of E. coli mutants deficient in AO or QS, respectively. Therefore, higher plants, such as Arabidopsis, appear to synthesize quinolinic acid by the E. coli-type pathway. To study subcelluar localization of quinolinate synthesis, AO and QS cDNA were fused to GFP and expressed in Arabidopsis plants. Expression of the GFP-tagged enzymes complemented lethality of respective T-DNA insertion mutants. GFP fluorescence was found in plastids in both transformants. These results suggested quinolinate is biosynthesize in plastids.
