SUMMARY: Ribulose 1:5-diphosphate carboxylase, which catalyses the conversion of ribulose 1:5-diphosphate and CO to 3-phosphoglyceric acid, is a key enzyme in the reductive pentose cycle. It was formed only in traces by growing in the dark under conditions of high or low aeration but organisms grown photosynthetically were rich in the enzyme. The activity of some other enzymes of the cycle, phosphoriboisomerase, phosphoglyceric acid kinase and triosephosphate dehydrogenase (diphosphopyridine nucleotide-linked) did not vary significantly with the growth conditions. The differential rate of synthesis (increase in enzyme/ increase in culture density) of the carboxylase was studied in cultures of growing exponentially under various conditions. It was influenced by the light intensity, being decreased when this was increased. Oxygen completely repressed formation of the enzyme even under continuous illumination; this effect was annulled by restoration of anaerobic conditions. Dark-grown organisms formed the enzyme at a high differential rate immediately on transfer to anaerobic+light conditions, provided that they contained a minimal amount of bacteriochlorophyll. Biotin deficiency did not affect the synthesis of the carboxylase. The conditions which promoted carboxylase formation were in many respects similar to those favourable to synthesis of enzymes required for the early stages of bacteriochlorophyll formation. The connexion of these observations with the regulation of pigment synthesis and with the possible physiological role of the enzyme in the Athiorhodaceae is discussed.


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