Three mitochondrial transporters of Saccharomyces cerevisiae are essential for ammonium fixation and lysine biosynthesis in synthetic minimal medium

The nuclear genes of Saccharomyces cerevisiae YHM2, ODC1 and ODC2 encode three transporters that are localized in the inner mitochondrial membrane. In this study, the roles of YHM2, ODC1 and ODC2 in the assimilation of nitrogen and in the biosynthesis of lysine have been investigated. Both the odc1δ. odc2δ double knockout and the yhm2δ mutant grew similarly as the YPH499 wild-type strain on synthetic minimal medium (SM) containing 2% glucose and ammonia as the main nitrogen source. In contrast, the yhm2δ. odc1δ. odc2δ triple knockout exhibited a marked growth defect under the same conditions. This defect was fully restored by the individual expression of YHM2, ODC1 or ODC2 in the triple deletion strain. Furthermore, the lack of growth of yhm2δ. odc1δ. odc2δ on 2% glucose SM was rescued by the addition of glutamate, but not glutamine, to the medium. Using lysine-prototroph YPH499-derived strains, the yhm2δ. odc1δ. odc2δ knockout (but not the odc1δ. odc2δ and yhm2δ mutants) also displayed a growth defect in lysine biosynthesis on 2% glucose SM, which was rescued by the addition of lysine and, to a lesser extent, by the addition of 2-aminoadipate. Additional analysis of the triple mutant showed that it is not respiratory-deficient and does not display mitochondrial DNA instability. These results provide evidence that only the simultaneous absence of YHM2, ODC1 and ODC2 impairs the export from the mitochondrial matrix of i) 2-oxoglutarate which is necessary for the synthesis of glutamate and ammonium fixation in the cytosol and ii) 2-oxoadipate which is required for lysine biosynthesis in the cytosol. Finally, the data presented allow one to suggest that the yhm2δ. odc1δ. odc2δ triple knockout is suitable in complementation studies aimed at assessing the pathogenic potential of human SLC25A21 (ODC) mutations.