Uridine 5'-phosphate (UMP) synthase is a multifunctional protein that contains the last two enzyme activities for the de novo biosynthesis of UMP, orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine-5'-phosphate (OMP) decarboxylase (EC 4.1.1.23). The native enzyme from mouse Ehrlich ascites cells exists in at least three distinct aggregation/conformation states as measured by sedimentation in sucrose gradients: a 3.6S monomer, a 5.1S dimer, and a conformationally altered 5.6S dimer. It has previously been reported that a variety of ligands (of which the most effective is OMP) mediate the conversion of the 3.6S monomer to the two types of dimers. Initial velocity studies with the enzyme in the different native states show that all three forms of UMP synthase have phosphoribosyltransferase activity but that the OMP decarboxylase is either uniquely or at least predominantly associated with the 5.6S form. Activation of this enzyme activity by the substrate appears to be the result of both a dimerization and a conformation step.