The synthesis of one-carbon units from CO2 in Clostridium kluyveri.

Abstract

Clostridium kluyveri was grown (a) on standard media, in which alternately [14C]bicarbonate, [U‐14C], [1‐14C], and 2‐14C]acetate plus ethanol were the labeled substrates, and (b) on standard media supplemented with either formate, glycine or serine, in which bicarbonate or formate were 14C‐labeled. Adenine, guanine and methionine were isolated from the cells and degraded.(a) Carbon atoms 2 and 8 of the purines were found to be synthesized from CO2 (about 70%) and from the methyl group (about 20%) and the carboxyl group (about 10%) of acetate. The S‐methyl group of methionine was shown to be derived almost equally from CO2 (about 50%) and from the two‐carbon substrates, the methyl group of acetate accounting for about 35% and the carboxyl group for about 15%.(b) The participation of CO2 in the synthesis of the S‐methyl group of methionine remained unaffected (about 50%) in the presence of serine, while it was decreased to below 5% in the presence of formate and increased to over 80% in the presence of glycine. The participation of formate was in the order of 55%.These data demonstrate that (1) two major competitive pathways for the synthesis of onecarbon units exist in Clostridium kluyveri: C1‐units are formed predominantly by a reduction of CO2 and to a minor degree by a cleavage of serine, the usual pathway (C3‐serine = C2‐acetate) and (2) the relative rates of the two pathways are controlled by the glycine requirement of the growing organism.