Investigation of the dark metabolism of acetate in photoheterotrophically grown cells of Rhodospirillum rubrum

The mechanism of the aerobic dark assimilation of acetate in the photoheter otrophically grown purple nonsulfur bacterium Rhodospirillum rubrum was stu died. Both in the light and in the dark, acetate assimilation in Rsp. rubru m cells, which lack the glyoxylate pathway, was accompanied by the excretio n of glyoxylate into the growth medium. The assimilation of propionate was accompanied by the excretion of pyruvate. Acetate assimilation was found to be stimulated by bicarbonate, pyruvate, the C-4-dicarboxylic acids of the Krebs cycle, and glyoxylate, but not by propionate. These data implied that the citramalate (CR;I) cycle in Rsp. rubrum cells can function as an anapl erotic pathway under aerobic dark conditions. This supposition was confirme d by respiration measurements. The respiration of cells oxidizing acetate d epended on the presence of CO2 in the medium. The fact that the intermediat es of the CM cycle (citramalate and mesaconate) markedly inhibited acetate assimilation but had almost no effect on cell respiration indicated that ci tramalate and mesaconate were intermediates of the acetate assimilation pat hway. The inhibition of acetate assimilation and cell respiration by itacon ate was due to its inhibitory effect on propionyl-CoA carboxylase, an enzym e of the CM cycle. The addition of 5 mM itaconate to extracts of Rsp. rubru m cells inhibited the activity of this enzyme by 85%. The data obtained sug gest that the CM cycle continues to function in Rsp. rubrum cells that have been grown anaerobically in the light and then transferred to the dark and incubated aerobically.