Incomplete Wood–Ljungdahl pathway facilitates one-carbon metabolism in organohalide-respiring Dehalococcoides mccartyi

Significance We have studied the functionality of an incomplete acetyl-CoA “Wood–Ljungdahl” pathway in a strictly organohalide-respiring bacterium, Dehalococcoides mccartyi. We found that in addition to its ability to incorporate exogenous formate, this pathway cleaves acetyl-CoA to generate methyl-tetrahydrofolate for methionine biosynthesis, serving as a unique substitute of the missing methylene-tetrahydrofolate reductase function. We also found that accumulation of carbon monoxide (CO), an obligate by-product from acetyl-CoA cleavage, inhibits D. mccartyi axenic cultures, but can be ameliorated by the presence of a CO-oxidizing organism, resulting in an unusual syntrophic association. The understanding of the products and biosynthetic functions of this incomplete Wood–Ljungdahl pathway improves our knowledge of alternate central metabolic strategies used by environmental microorganisms. The acetyl-CoA “Wood–Ljungdahl” pathway couples the folate-mediated one-carbon (C1) metabolism to either CO2 reduction or acetate oxidation via acetyl-CoA. This pathway is distributed in diverse anaerobes and is used for both energy conservation and assimilation of C1 compounds. Genome annotations for all sequenced strains of Dehalococcoides mccartyi, an important bacterium involved in the bioremediation of chlorinated solvents, reveal homologous genes encoding an incomplete Wood–Ljungdahl pathway. Because this pathway lacks key enzymes for both C1 metabolism and CO2 reduction, its cellular functions remain elusive. Here we used D. mccartyi strain 195 as a model organism to investigate the metabolic function of this pathway and its impacts on the growth of strain 195. Surprisingly, this pathway cleaves acetyl-CoA to donate a methyl group for production of methyl-tetrahydrofolate (CH3-THF) for methionine biosynthesis, representing an unconventional strategy for generating CH3-THF in organisms without methylene-tetrahydrofolate reductase. Carbon monoxide (CO) was found to accumulate as an obligate by-product from the acetyl-CoA cleavage because of the lack of a CO dehydrogenase in strain 195. CO accumulation inhibits the sustainable growth and dechlorination of strain 195 maintained in pure cultures, but can be prevented by CO-metabolizing anaerobes that coexist with D. mccartyi, resulting in an unusual syntrophic association. We also found that this pathway incorporates exogenous formate to support serine biosynthesis. This study of the incomplete Wood–Ljungdahl pathway in D. mccartyi indicates a unique bacterial C1 metabolism that is critical for D. mccartyi growth and interactions in dechlorinating communities and may play a role in other anaerobic communities.