Nitrite reductase activity in F-dependent sulphite reductase (Fsr) from .

(), a hyperthermophilic and evolutionarily deeply rooted methanogenic archaeon from a deep-sea hydrothermal vent, produces F-dependent sulphite reductase (Fsr) in response to exposure to sulphite. This enzyme allows to detoxify sulphite, a potent inhibitor of methyl coenzyme-M reductase (Mcr), by reducing it to sulphide with reduced coenzyme F (FH) as an electron donor; Mcr is essential for energy production for a methanogen. Fsr allows to utilize sulphite as a sulphur source. Nitrite is another potent inhibitor of Mcr and is toxic to methanogens. It is reduced by most sulphite reductases. In this study, we report that Fsr reduced nitrite to ammonia with FH with physiologically relevant values (nitrite, 8.9 µM; FH, 9.7 µM). The enzyme also reduced hydroxylamine with a value of 112.4 µM, indicating that it was an intermediate in the reduction of nitrite to ammonia. These results open the possibility that could use nitrite as a nitrogen source if it is provided at a low concentration of the type that occurs in its habitat.