Two phenotypically compensating isocitrate dehydrogenases in Ralstonia eutropha

The tricarboxylic acid (TCA) cycle enzyme isocitrate dehydrogenase (IDH) and the glyoxylate bypass enzyme isocitrate lyase are involved in catabolism of isocitrate and play a key role in controlling the metabolic flux between the TCA cycle and the glyoxylate shunt. Two IDH genes icd1 and icd2 of Ralstonia eutropha HF39, encoding isocitrate dehydrogenase 1 (IDH1) and isocitrate dehydrogenase 2 (IDH2), were identified and characterized. Icd1 was functionally expressed in Escherichia coli, whereas icd2 was expressed in E. coli but no activity was obtained. Interposon-mutants of icd1 (HF39Δ icd1) and icd2 (HF39Δ icd2) of R. eutropha HF39 were constructed and their phenotypes were investigated. HF39Δ icd1 retained 43% of IDH activity, which was not induced by acetate, and HF39Δ icd2 expressed 74% of acetate-induced IDH activity. Both HF39Δ icd1and HF39Δ icd2 kept the same growth rate on acetate as the wild-type. These data suggested that IDH1 is induced by acetate. The interposon-mutants HF39Δ icd1 and HF39Δ icd2 accumulated the same amount of poly(3-hydroxybutyric acid) as the wild-type.