Genetic, Biochemical, and Molecular Characterization of Methanosarcina barkeri Mutants Lacking Three Distinct Classes of Hydrogenase

Abstract

The methanogenic archaeon Methanosarcina barkeri encodes three distinct types of hydrogenase, whose functions vary depending on the growth substrate. These include the F_(420)-dependent (Frh), methanophenazine-dependent (Vht), and ferredoxin-dependent (Ech) hydrogenases. To investigate their physiological roles, we characterized a series of mutants lacking each hydrogenase in various combinations. Mutants lacking Frh, Vht, or Ech in any combination failed to grow on H_2-CO_2, whereas only Vht and Ech were essential for growth on acetate. In contrast, a mutant lacking all three grew on methanol with a final growth yield similar to that of the wild type and produced methane and CO2 in the expected 3:1 ratio but had a ca. 33% lower growth rate. Thus, hydrogenases play a significant, but nonessential, role during growth on this substrate. As previously observed, mutants lacking Ech failed to grow on methanol-H_2 unless they were supplemented with biosynthetic precursors. Interestingly, this phenotype was abolished in the Δech Δfrh and Δech Δfrh Δvht mutants, consistent with the idea that hydrogenases inhibit methanol oxidation in the presence of H_2, which prevents production of the reducing equivalents needed for biosynthesis. Quantification of the methane and CO_2 produced from methanol by resting cell suspensions of various mutants supported this conclusion. On the basis of the global transcriptional profiles, none of the hydrogenases were upregulated to compensate for the loss of the others. However, the transcript levels of the F_(420) dehydrogenase operon were significantly higher in all strains lacking frh, suggesting a mechanism to sense the redox state of F_(420). The roles of the hydrogenases in energy conservation during growth with each methanogenic pathway are discussed.