Catalyzes the reduction of sulfite to sulfide using reduced F420 as the electron source. Involved in sulfite detoxification and assimilation. Cannot use NADH or NADPH. 2 H(+) + 3 H2O + hydrogen sulfide + 3 oxidized coenzyme F420-(gamma-L-Glu)(n) = 3 reduced coenzyme F420-(gamma-L-Glu)(n) + sulfite Binds 4 [4Fe-4S] cluster. Binds 1 siroheme per subunit. Optimum pH is 7.0. Optimum temperature is above 95 degrees Celsius. Induced by sulfite. Contains an N-terminal H(2)F420 dehydrogenase domain and a C-terminal dissimilatory-type siroheme sulfite reductase domain. Expression in Methanococcus maripaludis, a sulfite-sensitive methanogen, leads to sulfite detoxification and use of sulfite as a sole sulfur source by M. maripaludis. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family.