Sphingomonas sp. strain RW1 synthesized a constitutive enzyme system that oxygenated dibenzofuran (DBF) to 2,2',3-trihydroxybiphenyl (THB). We purified this dibenzofuran 4,4a-dioxygenase system (DBFDOS) and found it to consist of four components which catalyzed three activities. Two isofunctional, monomeric flavoproteins (components A1 and A2; M(r) of about 44,000) transferred electrons from NADH to the second component (B; M(r) of about 12,000), a ferredoxin, which transported electrons to the heteromultimeric (alpha 2 beta 2) oxygenase component (C; M(r) of alpha, 45,000; M(r) of beta, 23,000). DBFDOS consumed 1 mol each of NADH, O2, and DBF, which was dioxygenated to about 1 mol of THB; no intermediate was observed. The reaction was thus the dioxygenation of DBF at the 4 and 4a positions to give a diene-diol-hemiacetal which rearomatized by spontaneous loss of a phenolate group to form THB. Components A1 and A2 each reduced dichlorophenolindophenol but had negligible activity with cytochrome c; each lost the yellow color, observed to be flavin adenine dinucleotide, upon purification. Component B, which transported electrons to the oxygenase or cytochrome c, had an N-terminal amino acid sequence with high homology to the putidaredoxin of cytochrome P-450cam. The oxygenase had the UV spectrum of a Rieske iron-sulfur center. We presume DBFDOS to be a class IIA dioxygenase system (EC 1.14.12.-), functionally similar to pyrazon dioxygenase.