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. Provides DszA and DszC (DBTO2-monooxygenase and DBT-monooxygenase respectively) with reduced flavin (FMN). FMNH2 + NAD(+) = FMN + 2 H(+) + NADH Sulfur metabolism; dibenzothiophene degradation. Repressed by HBP or sulfate. Expression in B.subtilis confers the ability to remove sulfur from polycyclic aromatic sulfur compounds found in gasoline and diesel (biodesulfurization), which are a considerable source of pollution. Belongs to the non-flavoprotein flavin reductase family.