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Johnson, Emmett J. (University of Mississippi Medical Center, Jackson), and Harry D. Peck, Jr. Coupling of phosphorylation and carbon dioxide fixation in extracts of Thiobacillus thioparus. J. Bacteriol. 89:1041-1050. 1965.-A cell-free system from Thiobacillus thioparus which fixes large quantities of C(14)O(2) in the presence of ribose-5-phosphate, adenosine triphosphate (ATP), and Mg(++) has been described. The specific activity (0.041 mumole of ribulose-1,5-diphosphate min(-1) mg(-1) protein) of the CO(2)-fixing system approaches that of green plants, and is further evidence for the importance of the role of carboxydismutase in the thiobacilli. In addition to ATP, adenosine diphosphate (ADP) and other nucleoside triphosphates served with varying degrees of effectiveness for the fixation of C(14)O(2). The ATP requirement for CO(2) fixation was partially replaced under aerobic conditions by a combination of SO(3) (=), PO(4) ( identical with), and adenosine monophosphate (AMP). Phosphorylation and CO(2) fixation were separated in time by first incubating SO(3) (=) and AMP aerobically, and then anaerobically introducing C(14)O(3) (=) and ribose-5-phosphate into the reaction mixture. During the first incubation, P(32)O(4) ( identical with) was esterified into nucleotides, mainly ADP, and in the second incubation C(14)O(2) was fixed, with the concomitant utilization of almost equal amounts of the esterified phosphate. These data provide the first in vitro evidence for the mechanism of the coupling of CO(2) fixation and phosphorylation in T. thioparus. The fixation of C(14)O(2) was shown to be almost completely inhibited by AMP. This inhibition was not due to the conversion of ATP to ADP by adenylic kinase, or to the binding of magnesium by the nucleotide. The inhibition was specific for AMP, since other mononucleotides, adenosine, and adenine did not inhibit. The AMP regulation of CO(2) fixation may represent a basic control mechanism in autotrophic metabolism.

Ljungdahl, Lars (Western Reserve University, Cleveland, Ohio), and Harland G. Wood. Incorporation of C(14) from carbon dioxide into sugar phosphates, carboxylic acids, and amino acids by Clostridium thermoaceticum. J. Bacteriol. 89:1055-1064. 1965.-The mechanism of synthesis of acetate from carbon dioxide by Clostridium thermoaceticum was investigated by incubating cells with glucose or xylose in the presence of C(14)O(2). Sugar phosphates, amino acids, and carboxylic acids were isolated and the specific radioactivities were determined; the distributions of C(14) were also determined in some of the compounds. Only fructose-1,6-diphosphate, formate, and lactate had higher specific activities than the acetate. The specific activities and distribution of C(14) in the fructose-6-phosphate and ribose-5-phosphate were such that we conclude that the synthesis of acetate does not occur via a pathway involving the sugar phosphates as direct intermediates. Likewise, it is shown that pathways including lactate, aspartate, serine, glycine, malate, and succinate are not of importance in the synthesis of acetate from CO(2). The methyl group of free methionine was unlabeled and is not a precursor of the methyl group of acetate.