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The atmospheric concentration of the potent greenhouse gases methane and nitrous oxide (N2O) has increased drastically during the last century. Methylomirabilis bacteria can play an import role in controlling the emission of these two gases from natural ecosystems, by oxidizing methane to CO2 and reducing nitrite to N2 without producing N2O. These bacteria have an anaerobic metabolism, but are proposed to possess an oxygen-dependent pathway for the activation of methane. Methylomirabilis bacteria reduce nitrite to NO, and are proposed to dismutate NO into O2 and N2 by a putative NO dismutase (NO-D). The O2 produced in the cell can then be used for the activation of methane by a particulate methane monooxygenase. So far, the metabolic model of Methylomirabilis bacteria was based mainly on (meta)genomics and physiological experiments. Here we applied a complexome profiling approach to determine which of the proposed enzymes are actually expressed in Methylomirabilis lanthanidiphila. To validate the metabolic model, we focused on enzymes involved in respiration, and nitrogen and C1 transformation. All complexes proposed to be involved in nitrite-dependent methane oxidation, were identified in M. lanthanidiphila, including the putative NO-D. Furthermore, several complexes involved in nitrate reduction/nitrite oxidation and NO reduction were detected, which likely play a role in detoxification and redox homeostasis. In conclusion, complexome profiling validated the expression and composition of enzymes proposed to be involved in the energy, methane and nitrogen metabolism of M. lanthanidiphila, thereby further corroborating the metabolically unique and environmentally relevant process of nitrite-dependent methane oxidation.
