Carbon Dioxide Fixation as the Initial Step in the Metabolism of Acetone by Thiosphaera pantotropha

Evidence is presented for a new pathway of acetone metabolism in Thiosphaera pantotropha. The initial step involves a carboxylation, probably resulting in the formation of acetoacetate. Cells grown on acetone and propan-2-ol fixed large quantities of 14CO2, in contrast to cells grown on acetate and acetoacetate. Growth on acetone and propan-2-ol, but not on other substrates, was dependent on the exogenous supply of CO2. NMR studies on the labelling pattern of the intracellular poly-β-hydroxybutyrate (PHB) confirmed that C2 units are produced from acetone after initial carboxylation. The metabolism of acetone and propan-2-ol was associated with drastic changes in the ultrastructure of the organisms. During growth on these substrates, proteinaceous, crystalline inclusions were observed. These were absent in acetate-grown cells. The nature of these crystalline inclusions remains to be elucidated.