Uridine 5'-phosphate (UMP) synthase is multifunctional protein that contains the last two enzyme activities of the de novo pathway for UMP biosynthesis, orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine-5'-phosphate (OMP) decarboxylase (EC 4.1.1.23). We have previously reported that UMP synthase from mouse Ehrlich ascites cells can exist in at least three distinct aggregation and/or conformation states, as measured by changes in sedimentation through sucrose gradients [Traut, T. W., & Jones, M. E. (1979) J. Biol. Chem, 254, 1143-1150]. The major sedimenting species were a 3.6S monomer, a 5.1S dimer, and a 5.6S species. The formation of the 5.1S dimer from the 3.6S monomer occurs in the presence of ligands that are competitive inhibitors at the OMP decarboxylase catalytic site. This paper presents evidence for a regulatory site, distinct from either of the two catalytic sites, which appears to mediate the conversion of the 5.1S dimer to the 5.6S form upon binding certain pyrimidine nucleotides (OMP, UMP, and 6-azaUMP). Since UMP synthase sediments predominantly as a dimer in the presence of substrates, regulation of the aggregation/conformation state of this multifunctional protein may be physiologically significant.