THE MECHANISM OF ACETATE ASSIMILATION IN THE PURPLE NONSULFUR BACTERIUM RHODOSPIRILLUM-RUBRUM LACKING ISOCITRATE LYASE

Rhodospirillum rubrum is assumed to have a hitherto unknown anapleroti c cycle of acetate assimilation, which may explain the ability of this bacterium, known to lack the glyoxylate cycle, to grow on acetate as the sole organic substrate. The initial stage of this cycle is the con version of acetate and pyruvate into glyoxylate and propionate via cit ramalate as an intermediate. Pyruvate is regenerated from propionate i n reactions catalyzed by propionyl-CoA carboxylase, succinate dehydrog enase, fumarate hydratase, malate dehydrogenase, phosphoenolpyruvate c arboxylase, and pyruvate kinase. Synthesis of these enzymes in Rsp. ru brum is constitutive. The overall reaction of the suggested citramalat e cycle is acetate --> glyoxylate + 4[H]. The main function of this cy cle is the oxidation of acetate to glyoxylate. Pyruvate synthase is no t involved in acetate assimilation in Rsp. rubrum.