A PROPOSED CITRAMALATE CYCLE FOR ACETATE ASSIMILATION IN THE PURPLE NONSULFUR BACTERIUM RHODOSPIRILLUM-RUBRUM

During phototrophic growth on acetate and CO2 Rhodospirillum rubrum 2R contained malate synthase but lacked isocitrate lyase. Acetate assimi lation by R. rubrum cells was stimulated by pyruvate, propionate glyox ylate, CO2 and H-2. Acetate photoassimilation by R. rubrum cells in th e presence of bicarbonate was accompanied by glyoxylate secretion, whi ch increased after addition of fluoroacetate and decreased after addit ion of malonate. When acetyl-CoA was incubated with pyruvate in cell-f ree extracts, citramalate was formed. Citramalate was also formed from propionyl-CoA and glyoxylate. The existence in R. rubrum of a CO2-dep endent cyclic pathway of acetate oxidation to glyoxylate with citramal ate as an intermediate is proposed. Inhibitor analysis of acetate and bicarbonate assimilation indicated that pyruvate synthase is not invol ved in acetate assimilation in R. rubrum. The possible anaplerotic seq uences employed by R. rubrum during phototrophic growth on acetate are discussed.