The NH(3)-dependent NAD(+) synthetase (NADS, gene nadE) participates in the biosynthesis of nicotinamide adenine dinucleotide (NAD(+)) by transforming nicotinic acid adenine dinucleotide (NaAD) to NAD(+). This enzyme is conserved in nearly all bacterial pathogens and a promising drug target for the development of new antibiotics. The crystal structure of NADS from Campylobacter jejuni subsp. jejuni NCTC 11168 in complex with the nitrate ion was determined at 2.74 A resolution. The structure accommodates a tight homodimer with a remarkable alpha/beta subunit topology. The major loop (178-196) that has a role in substrate recognition and stabilization, in addition to the protection of the reaction intermediate is disordered in the active site in both subunits of the dimer molecule. The nitrate ion is found in a splay between strands b1 and b2 where adenine group of AMP molecule is bound for catalysis in structures of known NADS. Our data is the first to suggest the nitrate ion binding site in NADS enzymes.