Synthesizing and salvaging NAD+: Lessons learned from Chlamydomonas reinhardtii

The essential coenzyme nicotinamide adenine dinucleotide (nad+) plays important roles in metabolic reactions and cell regulation in all organisms. bacteria,fungi,plants,and animals use different pathways to synthesize nad+. our molecular and genetic data demonstrate that in the unicellular green alga chlamydomonas nad+ is synthesized from aspartate (de novo synthesis),as in plants,or nicotinamide,as in mammals (salvage synthesis). the de novo pathway requires five different enzymes: l-aspartate oxidase (aso),quinolinate synthetase (qs),quinolate phosphoribosyltransferase (qpt),nicotinate/ nicotinamide mononucleotide adenylyltransferase (nmnat),and nad+ synthetase (ns). sequence similarity searches,gene isolation and sequencing of mutant loci indicate that mutations in each enzyme result in a nicotinamide-requiring mutant phenotype in the previously isolated nic mutants. we rescued the mutant phenotype by the introduction of bac dna (nic2- 1 and nic13-1) or plasmids with cloned genes (nic1-1 and nic15-1) into the mutants. nmnat,which is also in the de novo pathway,and nicotinamide phosphoribosyltransferase (nampt) constitute the nicotinamide-dependent salvage pathway. a mutation in nampt (npt1-1) has no obvious growth defect and is not nicotinamide-dependent. however,double mutant strains with the npt1-1 mutation and any of the nic mutations are inviable. when the de novo pathway is inactive,the salvage pathway is essential to chlamydomonas for the synthesis of nad+. a homolog of the human sirt6-like gene,srt2,is upregulated in the ns mutant,which shows a longer vegetative life span than wild-type cells. our results suggest that chlamydomonas is an excellent model system to study nad+ metabolism and cell longevity. © 2010 lin et al.