The stereospecificity of the citrate synthase in sulfate-reducing and photosynthetic bacteria.

Abstract

The stereospecificity of the citrate synthase of a number of sulfate-reducing and photosynthetic bacteria was determined by the following method: radioactive citrate was synthesized from [4-14C]oxaloacetate and acetylcoenzyme A by cell-free extracts. The resulting citrate was then cleaved to acetate and malate by citrate lyase plus malate dehydrogenase. In the case that the usual citrate synthase was present in the cell-free extracts, the citrate cleavage yielded radioactive malate and unlabeled acetate. The presence of the Clostridium kluyveri type of citrate synthase led to the formation of radioactive acetate and unlabeled malate. It was found that four species of the Athiorhodaceae, Desultfovibrio gigas, D. sale xigens and Desulfotomaculum ruminis contain the usual citrate synthase. Two strains of D. desulfuricans and two strains of D. vulgaris contain the C. kluyveri type of citrate synthase, which is now referred to as (R)-citrate synthase. The presence of the (R)-citrate synthase in D. desulfuricans and D. vulgaris was confirmed by an in-vivo experiment. When grown on pyruvate and radioactive carbon dioxide, the protein of these two species contains [5-14C]glutamate, whereas the protein of D. gigas contains [1-14C]-glutamate. The presence of the (R)-citrate synthase in certain strictly anaerobic bacteria is discussed from an evolutionary point of view.