Unraveling methyl chloride conversion in the acetogen Acetobacterium dehalogenans

Methyl chloride (MC) stands out as the most prevalent organochlorine in the atmosphere, contributing 15-20% to chlorine-catalyzed ozone depletion in the stratosphere [1]. The acetogenic bacterium Acetobacterium dehalogenans utilizes MC as its sole energy source [2]. While the methyltransferase systems of A. dehalogenans for the demethylation of methoxylated aromatics have been characterized [3], the system responsible for MC demethylation remained unidentified. This study aims to understand the metabolic changes of A. dehalogenans during growth on MC versus the methoxylated aromatic syringate and to identify the enzyme system catalyzing MC demethylation/dehalogenation. We analyzed the growth and substrate conversion of A. dehalogenans grown on MC, syringate and both substrates. Furthermore, we used comparative transcriptomics to analyze the gene expression of A. dehalogenans grown under these conditions. We found more than 100 highly differentially expressed genes (log2FC >3) comparing MC vs. syringate as substrate (67 genes up-, 78 downregulated) and syringate plus MC vs. syringate (164 genes up-, 43 downregulated). During growth on MC, three neighboring genes were highly upregulated (log2FC >10) that encode a yet uncharacterized corrinoid-dependent methyltransferase system. This system is most probably responsible for the transfer of the methyl group from MC to tetrahydrofolate, a C1 carrier in the Wood-Ljungdahl pathway. We are heterologously producing and purifying the corresponding proteins of A. dehalogenans in E. coli and purifying them and will subsequently perform UV-Vis spectroscopy as well as activity assays to define substrate utilization and reaction kinetics. Blast analysis further indicates that homologous genes are present in several other anaerobic bacteria, which may enable these microorganisms to convert MC by a similar enzyme system.

[1] Harper, D. B. (2000). Nat Prod Rep, 17, 337-348.

[2] Traunecker, J., et al. (1991). Arch Microbiol, 156, 416–421.

[3] Engelmann, T. et al. (2001). Arch Microbiol, 175, 376–383.