Ectopic expression of cytosolic DHODH uncouples de novo pyrimidine biosynthesis from mitochondrial electron transport.

Dihydroorotate dehydrogenase is a rate-limiting enzyme of de novo pyrimidine synthesis. In most eukaryotes, this enzyme is bound to the inner mitochondrial membrane, where it couples orotate synthesis to ubiquinone reduction. As ubiquinone must be regenerated by respiratory complex III, pyrimidine biosynthesis and cellular respiration are tightly coupled. Consequently, inhibition of respiration suppresses DNA synthesis and cell proliferation. Here we show that expression of the Saccharomyces cerevisiae URA1 gene (ScURA) in mammalian cells uncouples pyrimidine biosynthesis from mitochondrial electron transport. ScURA forms a homodimer in the cytosol that uses fumarate as an electron acceptor instead of ubiquinone, enabling respiration-independent pyrimidine biosynthesis. Cells expressing ScURA are resistant to drugs that inhibit complex III and the mitochondrial ribosome. Additionally, ScURA enables growth of mitochondrial-DNA-lacking ρ(0) cells in uridine-deficient medium and ameliorates the phenotype of cellular models of mitochondrial diseases. Overall, this genetic tool uncovers the contribution of pyrimidine biosynthesis to the phenotypes arising from electron transport chain defects.