A New Glance on the Mechanism of Autotrophic CO2 Assimilation in Green Sulfur Bacteria

— The autotrophic system of CO 2 fixation in Chlorobaculum limnaeum strain C consists of two topologically independent enzyme complexes: the reverse tricarboxylic acid cycle (complex I) with acetyl-CoA as the only product, and a noncyclic complex of reactions (complex II). In the reactions catalyzed by the complex II enzymes, acetyl-CoA synthesized in the reverse TCA cycle is used to synthesize all the substrates required for biomass production. Complex I includes two carboxylation reactions catalyzed by 2-oxoglutarate synthase and isocitrate dehydrogenase. Complex II includes two additional carboxylation reactions, which are catalyzed by pyruvate synthase and PEP carboxylase. The enzyme complexes I and II are united into a system of autotrophic CO 2 assimilation by citrate synthase, which is not present in the classical variant of the Arnon‒Buchanan cycle. The citrate synthase gene was detected in all studied green sulfur bacteria and may be considered a housekeeping gene.