Mutations in theIDH2Gene Encoding the Catalytic Subunit of the Yeast NAD+-Dependent Isocitrate Dehydrogenase Can Be Suppressed by Mutations in theCIT1Gene Encoding Citrate Synthase and Other Genes of Oxidative Metabolism

During a screen for respiration competent yeast mutants that were unable to grow with acetate as a carbon source, twoidh2 cit1double mutants were identified. These strains were defective in the catalytic subunit of the NAD+-dependent isocitrate dehydrogenase and citrate synthase of the tricarboxylic acid (TCA) cycle. The strains harboring theidh2alleles from these strains had two unusual phenotypes. First, their growth on many nonfermentable carbon sources was much poorer than strains containing otheridh2mutations. Second, the poor growth phenotype could be suppressed by the presence of mutations inCIT1and other genes encoding oxidative functions. Spontaneous suppressor mutants that restore fast growth on glycerol medium to strains harboring twoidh2alleles were isolated, and a large percentage of the suppressor mutations have been identified within theCIT1gene and at several other loci. Elevated levels of several TCA cycle proteins were observed in theseidh2mutants that were not observed in the presence of suppressingcit1mutations. Citrate and isocitrate concentrations were also elevated in theidh2mutants, but probably not to toxic levels. Fiveidh2alleles were sequenced to understand the defects of the two classes of mutations. Sequence analysis indicated that the poor growth phenotype was caused by the loss of Idh2p protein. Similarly, eightcit1alleles were sequenced to understand their characteristics as glycerol suppressors ofidh2.These and other studies indicate that any mutation withinCIT1was capable of suppressing theidh2mutations. Several models to explain these interactions are discussed.