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) in a stepwise manner (Ref.2, PubMed:11229908). Also acts on thioxanthen-9-one and 4,6-dimethyl DBT and 2,8-dimethyl DBT (Ref.2). 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. DBT degradation completely inhibited by Cu(2+), Mn(2+), p-chloromercuribenzoic acid, 2,2-bipyridyl, 1,10-phenanthroline, and strongly inhibited by Zn(2+), 5,5'- Dithiobis(2-nitrobenzoic acid) and 8-quinolinol. Optimum pH is 8.0. Optimum temperature is 40 degrees Celsius. Sulfur metabolism; dibenzothiophene degradation. Homotetramer formed by a dimer of dimers; FMN binds between monomers of the homodimer. Has 3 domains, the helical N-terminus (residues 18-124), a beta-barrel central domain (125-233) and a helical C-terminus (234-417). The lid loop (residues 131-142) change conformation when FMN is bound and close the FMN-binding site. Other regions that probably contribute to FMN binding are residues 178-184 and 281-291. Belongs to the DszC flavin monooxygenase family.