SUMMARY: The concentration of ribulose 1,5-diphosphate (RuDP) carboxylase, the enzyme which catalyses the conversion of ribulose 1,5-diphosphate + CO to 3-phosphoglyceric acid, was partially repressed in some Thiorhodaceae organisms when these were grown on certain organic compounds. Transfer of thiosulphate-grown organisms possessing a high concentration of enzyme into growth medium containing pyruvate caused a rapid decline in carboxylase activity. In the reverse situation, pyruvate-grown organisms preferentially synthesized RuDP carboxylase when transferred to growth medium containing thiosulphate alone. The presence of thiosulphate prevented loss of carboxylase with pyruvate. The incorporation pattern of CO into the ethanol-soluble compounds of organisms metabolizing thiosulphate alone was typical of autotrophic metabolism; most of the CO was fixed the reductive pentose cycle. The pattern of incorporation of CO by organisms metabolizing pyruvate was strikingly different in that CO entered the cell constituents predominantly a carboxylation leading to a four-carbon product. However, even under the latter conditions, the RuDP carboxylase and the reductive pentose cycle appeared to operate to some extent, since phosphoglycerate was an early product of CO fixation. Phosphoglycerate was an early product of CO fixation by thiosulphate-grown organisms incubated with a variety of organic substrates, showing that the carboxylase and the reductive pentose cycle could function under these conditions. The addition of thiosulphate increased incorporation of CO into phosphate esters by pyruvate-grown organisms incubated with pyruvate. It is concluded that ribulose diphosphate carboxylase and the reductive pentose cycle function in Thiorhodaceae even when grown on organic substrates; its quantitative importance was not assessed. The synthesis of the carboxylase was influenced by the growth substrate.


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