Orotate phosphoribosyltransferase (OPRTase) catalyzes the OMP-forming step in de novo pyrimidine-nucleotide biosynthesis. Here, the crystal structure of OPRTase from the caries pathogen Streptococcus mutans is reported at 2.4 A resolution. S. mutans OPRTase forms a symmetric dimer and each monomer binds two sulfates at the active sites. The structural symmetry of the sulfate-binding sites and the missing loops in this structure are consistent with a symmetric catalysis mechanism.