Quinolinate synthase (NadA) catalyzes a unique condensation reaction between dihydroxyacetone phosphate and iminoaspartate, yielding inorganic phosphate, 2 mol of water, and quinolinic acid, a central intermediate in the biosynthesis of nicotinamide adenine dinucleotide and its derivatives. The enzyme from Escherichia coli contains a C (291)XXC (294)XXC (297) motif in its primary structure. Bioinformatics analysis indicates that only Cys297 serves as a ligand to a [4Fe-4S] cluster that is required for turnover. In this report, we show that the two remaining cysteines, Cys291 and Cys294, undergo reversible disulfide-bond formation, which regulates the activity of the enzyme. This mode of redox regulation of NadA appears physiologically relevant, since disulfide-bond formation and reduction are effected by oxidized and reduced forms of E. coli thioredoxin. A midpoint potential of -264 +/- 1.77 mV is approximated for the redox couple.