Two genetically distinct methyl‐coenzyme M reductases in Methanobacterium thermoautotrophicum strain Marburg and ΔH

Methyl‐coenzyme M reductase (MCR) catalyzes the methane‐forming step in methanogenic archaebacteria. The reductase has been characterized in detail from Methanobacterium thermoautotrophicum strain Marburg and ΔH, which grow on H2 and CO2 as energy source. During purification of the enzyme we have now discovered a second methyl‐coenzyme M reductase (MCR II) in the two strains, which elutes at lower salt concentration from anion‐exchange columns than the enzyme (MCR I) previously characterized. MCR II is similar to MCR I in that it is also composed of three different subunits α, β, and γ but distinct from MCR I in that the γ subunit is 5 kDa smaller, as revealed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The N‐terminal amino acid sequences of the α, β, and γ subunits of MCR II and MCR I were found to be different in several amino acid positions. The respective sequences showed, however, strong similarities indicating that MCR II was not derived from MCR I by limited proteolysis. The relative amounts of MCR I and MCR II present in the cells were affected by the growth conditions. When the cultures were supplied with sufficient H2 and CO2 and the cells grew exponentially, essentially only MCR II was found. When growth was limited by the gas supply, MCR I predominated.