Ribulose Diphosphate Carboxylase in Thiorhodaceae

Abstract

The concentration of ribulose 1,5-diphosphate (RuDP) carboxylase, the enzyme which catalyzes the conversion of ribulose l,5-diphosphate+ CO2 to 3-phosphoglycerlc-acld, was partially repressed in some Thior-hodaceae organisms when these were grown on certain organic com-pounds. Transfer of thlosulphate-grown organisms possessing a high concentration of enzyme into growth medium containing pyruvate caused a rapid decline in carboxylase activity. fa the reverse situation, pyruvate-grown organisms preferentially synthesized RuDP carboxylase when transferred to growth medium containing thiosulphate alone. The presence of thlosulphate prevented loss of carboxylase with pyruvate. The incorporation pattern of C14O2 into the ethanol-soluble compounds of organisms metabolizing thiosulphate alone was typical of autotrophic metabolism; most of the CO2 was fixed via the reductive pentose cycle. The pattern of incorporation of C14O2 by organisms metabolizing pyruvate was strikingly different in that CO2 entered the cell constituents predominantly via 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 CO2 fixation. Phosphogly-cerate was an early product of C14O2 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 thlosulphate increased incorporation of C14O2 into phosphate esters by pyruvate-grown organisms incubated with pyruvate. It is concluded that rlbulose dlphosphate carboxylase and the reductive pentose cycle function in Thlorhodaceae even when grown on organic substrates; its quantitative importance was not assessed. The synthesis of the carboxylase was influenced by the growth substrate.