Two purified fractions from Clostridium thermoaceticum are shown to catalyze the following reaction: CO + CH3THF + CoA ATPCH3COCoA + THF. The methyltetrahydrofolate (CH3THF) gives rise to the methyl group of the acetylcoenzyme A (CoA) and the carbon monoxide (CO) and CoA to its carboxyl thio ester group. The role of ATP is unknown. One of the protein fractions (F2) is a methyltransferase, whereas the other fraction (F3) contains CO dehydrogenase and a methyl acceptor which is postulated to be a corrinoid enzyme. The methyltransferase catalyzes the transfer of the methyl group to the methyl acceptor, and the CO is converted to a formyl derivative by the CO dehydrogenase. By a mechanism that is as yet unknown, the formyl derivative in combination with CoA and the methyl of the methyl acceptor are converted to acetyl-CoA. It is also shown that fraction F3 catalyzes the reversible exchange of 14C from rl-14C]acetyl-CoA into 14CO and that ATP is required, but not the methyltransferase. It is proposed that these reactions are part of the mechanism which enables certain autotrophic bacteria to grow on CO. It is postulated that CH3THF is synthesized from CO and tetrahydrofolate which then, as described above, is converted to acetyl-CoA. The acetyl-CoA then serves as a precursor in other anabolic reactions. A similar autotrophic pathway may occur in bacteria which grow on carbon dioxide and hydrogen.The fermentation of glucose by Clostridium thermoaceticum occurs by a homoacetate pathway in which 3 mol of acetate is formed per mol of glucose; for a recent review, see Ljungdahl and Wood (22). The fermentation as outlined below in reactions 1 to 5 is considered to occur via the Embden-Meyerhof glycolytic pathway yielding 2 mol of pyruvate (reaction 1). One of the pyruvate molecules is converted to CO2 and acetate; the resulting CO2 is reduced to formate, which combines with tetrahydrofolate (THF) and is reduced to methyltetrahydrofolate (CH3THF) (reactions 2, 3, and 4). Thus, this portion of the pathway yields acetate and CH3THF. In the succeeding steps, the CH3THF and the second molecule of pyruvate are converted to two molecules of acetate (reaction 5).

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