Role of the reductive carboxylic acid cycle in a photosynthetic bacterium lacking ribulose 1,5-diphosphate carboxylase

Abstract

Evidence is presented that a green photosynthetic bacterium ( Chlorobium thiosulfatophilum, Tassajara) lacks ribulose 1,5-diphosphate carboxylase, the key enzyme of the reductive pentose phosphate cycle—the photosynthetic carbon reduction mechanism characteristic of green plants. The bacterium appears to use exclusively the reductive carboxylic acid cycle (and its associated reactions) not only, as previously recognized, in the photosynthetic conversion of CO 2 to amino acids and organic acids but also to carbohydrates. This conclusion is based on: (i) the absence in cell-free preparations of ribulose 1,5-diphosphate carboxylase; (ii) the assimilation by cell suspensions of 14CO 2, [ 14C]acetate, and [ 14C]succinate to give the products expected from the operation of the reductive carboxylic acid cycle; (iii) the demonstration in cell-free preparations of all enzymes needed for the conversion of CO 2 and the primary product of the reductive carboxylic acid cycle, acetyl-CoA, to carbohydrate. The results also show that CO 2 concentration influences the products formed by the reductive carboxylic acid cycle, with a low level of CO 2 favoring the synthesis of carbohydrates.