Orotidine monophosphate (OMP) decarboxylase catalyzes the final step in the de novo biosynthesis of uridine monophosphate. In most prokaryotes, OMP decarboxylase is a dimer of identical subunits, whereas in higher organisms, it is part of a bifunctional enzyme that also catalyzes the formation of OMP. The crystal structure of the apo-form Campylobacter jejuni orotidine 5'-monophosphate (OMP) decarboxylase was determined by molecular replacement and refined to an R-factor of 17.0% at 1.8 A resolution. There are two dimers in the asymmetric unit. Each monomer consists of a triosephosphate isomerase barrel and contains an active site that is located across one end of the barrel and near the dimer interface. Two iodide anions are bound at the active site in each of the four subunits. Surprisingly, Campylobacter jejuni and Bacillus subtilis OMP decarboxylases share only 31 % sequence identity, whereas they are very similar in structure with rmsd value of 1.7 A over 219 residues.