Evidence for the presence of the reductive pentose phosphate cycle in a filamentous anoxygenic photosynthetic bacterium, Oscillochloris trichoides strain DG-6

Studies on autotrophic CO2 fixation by the filamentous anoxygenic photosynt hetic bacterium Oscillochloris trichoides strain DG-6 demonstrated that, un like other green bacteria, this organism metabolized CO2 via the reductive pentose phosphate cycle. Both key enzymes of this cycle - ribulose1,5-bisph osphate carboxylase/oxygenase and phosphoribulokinase - were detected in ce ll extracts. The main product of ribulose 1,5-bisphosphate-dependent CO, fi xation was 3-phosphoglyceric acid. KCN, which is known to be a competitive inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase, completely in hibited the CO2 assimilation by whole cells as well as by cell extracts of O. trichoides. The C-13/C-12 carbon isotope fractionation during photoautot rophic growth of O. trichoides was -19.7%, which is close to that obtained for autotrophic organisms that use ribulose-1,5-bisphosphate carboxylase as the primary carboxylation enzyme. Cell extracts of O. trichoides contained all the enzymes of the tricarboxylic acid cycle except I-oxoglutarate dehy drogenase. No activity of isocitrate lyase, a key enzyme of the glyoxylate shunt, was found in cell extracts of O. trichoides DG-6.