Genetic and biochemical interactions involving tricarboxylic acid cycle (TCA) function using a collection of mutants defective in all TCA cycle genes

The eight enzymes of the tricarboxylic acid (TCA) cycle are encoded by at l east 15 different nuclear genes in Saccharomyces cerevisiae. We have constr ucted a set of yeast strains defective in these genes as part of a comprehe nsive analysis of the interactions among the TCA cycle proteins. The 15 maj or TCA cycle genes can be sorted into five phenotypic categories on the bas is of their growth on nonfermentable carbon sources. We have previously rep orted a novel phenotype associated with mutants defective in the IDH2 gene encoding the Idh2p subunit of the NAD(+)-dependent isocitrate dehydrogenase (NAD-IDH). Null and nonsense idh2 mutants grow poorly on glycerol, but gro wth can be enhanced by extragenic mutations, termed glycerol suppressors, i n the CIT1 gene encoding the TCA cycle citrate synthase and in other genes of oxidative metabolism. The TCA cycle mutant collection was utilized to se arch for other genes that can suppress idh2 mutants and to identify TCA cyc le genes that display a similar suppressible growth phenotype on glycerol. Mutations in 7 TCA cycle genes were capable of functioning as suppressors f or growth of idh2 mutants on glycerol. The only other TCA cycle gene to dis play the glycerol-suppressor-accumulation phenotype was IDH1, which encodes the companion Idhlp subunit of NAD-IDH. These results provide genetic evid ence that NAD-IDH plays a unique role in TCA cycle function.