Redox-active protein probably involved in electron transport during fermentation of acetate to methane. Binds 1 FMN per subunit. Binds 1 [4Fe-4S] cluster per subunit. Homodimer. The electrons derived from oxidation of the carbonyl group of acetate flow from ferredoxin to the 4Fe-4S cluster and then to the FMN cofactor. Belongs to the SsuE family. Isf subfamily.