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. 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. Sulfur metabolism; dibenzothiophene degradation. Homotetramer. Repressed by HBP or sulfate (Probable). Part of the dszA-dszB-dszC operon. This protein is expressed at high levels (at protein level) (PubMed:17420595). Has 3 domains, the helical N-terminus (residues 19-125), a beta-barrel central domain (126-234) and a helical C-terminus (235-409). The C-terminus (410-417) forms part of the substrate-binding pocket with the C-terminal helical domain (PubMed:24975806). The lid loop assumes one of 2 conformations allowing opening and closing of the active site (By similarity). 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 (PubMed:16810451). Modification of the operon so the start codon of dszB no longer overlaps with the stop codon of dszA leads to increased expression of DszB (in R.erythropolis) and about 5-fold higher levels of desulfurization of DBT (PubMed:17420595). Rearrangement of the operon into the order dszB-dszC-dszA leads to 12-fold higher levels of DBT desulfurization (PubMed:18165370). Belongs to the DszC flavin monooxygenase family.