Catalyzes the first step of the '4S' desulfurization pathway that removes covalently bound sulfur from dibenzothiophene (DBT) without breaking carbon-carbon bonds. Sulfur dioxygenase which converts DBT to DBT-sulfone (DBTO2 or DBT 5,5-dioxide) probably in a stepwise manner. In addition to FMNH2 can also use FAD (although FAD is less efficient). dibenzothiophene + 2 FMNH2 + 2 O2 = dibenzothiophene 5,5-dioxide + 2 FMN + 2 H(+) + 2 H2O dibenzothiophene + FMNH2 + O2 = dibenzothiophene 5-oxide + FMN + H(+) + H2O dibenzothiophene 5-oxide + FMNH2 + O2 = dibenzothiophene 5,5-dioxide + FMN + H(+) + H2O Reduced flavin is provided by flavin reductase DszD; in a coupled DszC-DszD reaction both FMNH2 and FADH2 can be used, maximal DBTO2 production is seen with 5 uM FMN or with 35 uM FAD. Inhibited at high concentrations of FMN or FAD. Sulfur metabolism; dibenzothiophene degradation. Homotetramer. The lid loop assumes one of 2 conformations allowing opening and closing of the active site. Belongs to the DszC flavin monooxygenase family.