An NADH:FMN oxidoreductase which supplies reduced FMN for the '4S' desulfurization pathway that removes covalently bound sulfur from dibenzothiophene (DBT) without breaking carbon-carbon bonds (PubMed:9634856, PubMed:20349330). Provides DszA and DszC (DBTO2-monooxygenase and DBT-monooxygenase respectively) with reduced flavin (FMN); has no activity on NADPH or FAD (PubMed:9634856). The pathway substrate specificity has been augmented using mutagenesis, however no mutations allowed use of alkylated thiophenes (PubMed:11823208). FMNH2 + NAD(+) = FMN + 2 H(+) + NADH Sulfur metabolism; dibenzothiophene degradation. In association with the dszA-dszB-dsz operon can be used to remove sulfur from polycyclic aromatic sulfur compounds found in gasoline and diesel (biodesulfurization), which are a considerable source of pollution. In addition it may be possible to engineer the operon to make specialty chemicals (PubMed:11823208). Can be engineered to make it more active, suggesting it may be possible increase the yield and rate of the biodesulfurization process (PubMed:20349330). Belongs to the non-flavoprotein flavin reductase family.