A Novel Coenzyme F420 Dependent Sulfite Reductase and a Small Sulfite Reductase in Methanogenic Archaea

Abstract

Recently a novel, highly active, coenzyme F-420 dependent sulfite reductase (Fsr) has been discovered in Methanocaldococcus jannaschii. Three other extremophilic methanogens and an uncultured archaeon from a consortium performing anaerobic oxidation of methane (AOM) carry Fsr homologs. Methanogens require sulfide and most are sensitive to sulfite. Since Fsr is induced by sulfite, reduces sulfite to sulfide with H2F420, and seems to be associated with the membrane, it is a sulfite detoxification and assimilation enzyme. The N-terminal half of Fsr is a homolog of H2F420 dehydrogenase (FqoF/FpoF). FqoF/FpoF is the electron input unit of a membrane-bound electron transport system of late-evolving methylotrophic methanogens and Archaeoglobus fulgidus, a sulfate reducing archaeon employing the partial reverse methanogenesis pathway. The C-terminal half (Fsr-C) represents a dissimilatory sulfite reductase subunit (DsrA). While only four methanogens carry Fsr, every methanogen carries a small putative sulfite reductase with sequence features of Fsr-C. These observations lead to following hypotheses. At one time methanogenesis and sulfate reduction involving a sulfite reductase, two of the oldest energy-conserving respiratory metabolisms of Earth, existed in one organism that performed sulfate reduction driven AOM. Fsr gave rise to FqoF/FpoF and DsrA, or from a small sulfite reductase of methanogens DsrA and Fsr (a fusion with FqoF/FpoF) evolved.