Regulation of Pyruvate Dehydrogenase Complex Synthesis in Escherichia coli K12. Identification of the Inducing Metabolite

1 The synthesis of the pyruvate dehydrogenase complex in Escherichia coli K12 is inducible, and pyruvate is the metabolite causing induction. In a mutant lacking the activities of phosphoenol pyruvate synthase and dihydrolipoamide transacetylase (a component of the pyruvate dehydrogenase complex) pyruvate is no longer significantly metabolized under certain growth conditions. In such a mutant pyruvate as well as α-ketobutyrate induces pyruvate dehydrogenase synthesis. Decreasing inducing activity was found with increasing chain length (or branching) of homologous α-ketoacids. Apo-pyruvate dehydrogenase is produced when thiamine requiring strains are grown in the absence of thiamine, i. e., the pyruvate dehydrogenase complex formed is inactive. Such strains, possessing wild type pyruvate dehydrogenase complex, accumulate pyruvate when growing without thiamine. Only with thiamine starvation could the synthesis of wild type pyruvate dehydrogenase complex be fully induced. Also, in merodiploid strains homogenotic for the wild type ace locus a gene dosage effect was found only upon thiamine deprivation. Inducibility is thus separable from enzymatic activity; enzymatically active pyruvate dehydrogenase complex removes the effector inducing its synthesis thus preventing full induction during normal growth conditions. No evidence was found for an indirect action by pyruvate, i. e., pyruvate does not induce by inhibition or activation of some other reaction (in the citric acid cycle or glycolysis), thereby causing an increased concentration of another metabolite which is the true inducer. 2 It is possible that pyruvate serves not only as inducing metabolite for pyruvate dehydrogenase complex synthesis but also as repressing metabolite for isocitric lyase synthesis (or for a glyoxylate cycle operon). 3 Both ace mutants and wild type possess pyruvate oxidase, an acetate producing enzyme system independent of the pyruvate dehydrogenase complex. It appears that the activity of the oxidase is too low to provide enough acetate which can replace that normally produced via pyruvate dehydrogenase complex. A physiological role for the oxidase has not been found.