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. Can be used to remove sulfur from polycyclic aromatic sulfur compounds found in gasoline and diesel (biodesulfurization), which are a considerable source of pollution. As the substrates are not very soluble in conventional media, biphasic systems may help improve catalysis. Expression of dszD-dszA-dszB-dszC (cloned in this order) in organic-solvent-tolerant P.putida strain Idaho allows P.putida to grow on 10% p-xylene with DBT as the sole sulfur source. In this P.putida strain 97% of DBT was degraded, 71% of 4,6-dimethyldibenzothiophene and about 50% of 3-methyldibenzothiophene or 4-methyldibenzothiophene was degraded in the presence of 10% p-xylene. Degradation of DBT in the presence of 10% of other organic solvents was tested; when grown in dodecane, cyclohexane or heptanol bacteria metabolized DBT as well as p-xylene, while other organic solvents degraded DBT slightly less well. Belongs to the non-flavoprotein flavin reductase family.