Two F420-reducing hydrogenases in methanosarcina barkeri

F420-reducing hydrogenases are nickel iron-sulfur flavoproteins involved in CO2 reduction with H2 to methane in methanogenic archaea. Evidence is presented that Methanosarcina barkeri contains two isoenzymes for which the encoding genes have been cloned and sequenced. The genes are organized in two operons, frhADGB and freAEGB, each comprising four open reading frames. Transcription analysis revealed that both operons are transcribed during growth of Ms. barkeri on H2/CO2, on methanol, and on trimethylamine, but not during growth on acetate.