Loss of the mtr operon in Methanosarcina blocks growth on methanol, but not methanogenesis, and reveals an unknown methanogenic pathway

In the methanogenic archaeon Methanosarcina barkeri Fusaro, the N⁵-methyl-tetrahydrosarcinapterin (CH₃-H₄SPT):coenzyme M (CoM) methyltransferase, encoded by the mtr operon, catalyzes the energy-conserving (sodium-pumping) methyl transfer from CH₃-H₄SPT to CoM during growth on H₂/CO₂ or acetate. However, in the disproportionation of C-1 compounds, such as methanol, to methane and carbon dioxide, it catalyzes the reverse, endergonic transfer from methyl-CoM to H₄SPT, which is driven by sodium uptake. It has been proposed that a bypass for this energy-consuming reaction may occur via a direct methyl transfer from methanol to H₄SPT. To test this, an mtr deletion mutant was constructed and characterized in M. barkeri Fusaro. The mutant is unable to grow on methanol, acetate or H₂/CO₂, but can grow on methanol with H₂/CO₂ and, surprisingly, methanol with acetate. ¹³C labeling experiments show that growth on acetate with methanol involves a previously unknown methanogenic pathway, in which oxidation of acetate to a mixture of CO₂ and formic acid is coupled to methanol reduction. Interestingly, although the mutant is unable to grow on methanol alone, it remains capable of producing methane from this substrate. Thus, the proposed Mtr bypass does exist, but is unable to support growth of the organism.