Evidence that a Single Polypeptide Catalyses the Two Step Conversion of Orotate to UMP in Cells from a Tomato...

A single polypeptide catalysing the two step conversion of orotate to UMP in cultured tomato cells was purified to near homogeneity as judged by analytical disc gel electrophoresis. After electrophoresis of the dodecyl sulphate denaturated enzyme one single protein band appeared from which both enzyme activities could be renaturated by addition of Triton X-100. As introduced by McClard et al. (Biochemistry 19, 4699-4706, 1980) for the mammalian system, this plant enzyme should be termed UMP synthase, consequently. The enzyme consists of a single polypeptide chain of a molecular weight of about 51,000. Molecular weight determination by gel filtration under non-denaturating conditions gave a value of about 100,000, suggesting dimer formation in vivo. The enzyme contains thiol groups essential for enzyme activity. Kinetic characteristics of the orotate phosphoribosyltransferase activity are: pH optimum 8.0, Km values for orotate and PRPP of 4.5 and 5.4µM, respectively. Orotidine-5'-phosphate decarboxylase activity is optimal between pH 7.2 and 8.5, the Km value for orotidine-5'-phosphate 2 µM.