Aromatic ring hydroxylating dioxygenases are multicomponent 1,2-dioxygenase complexes that convert closed-ring structures to non-aromatic cis-diols [<cite idref="PUB00002151"/>]. The complex has both hydroxylase and electron transfer components. The hydroxylase component is itself composed of two subunits: an alpha-subunit of about 50 kDa, and a beta-subunit of about 20 kDa. The electron transfer component is either composed of two subunits: a ferredoxin and a ferredoxin reductase or by a single bifunctional ferredoxin/reductase subunit. Sequence analysis of hydroxylase subunits of ring hydroxylating systems (including toluene, benzene and napthalene 1,2-dioxygenases) suggests they are derived from a common ancestor [<cite idref="PUB00002151"/>]. The alpha-subunit binds both a Rieske-like 2Fe-2S cluster and an iron atom: conserved Cys and His residues in the N-terminal region may provide 2Fe-2S ligands, while conserved His and Tyr residues may coordinate the iron. The beta subunit may be responsible for the substrate specificity of the dioxygenase system [<cite idref="PUB00002151"/>]. Aromatic-ring-hydroxylating dioxygenase, alpha subunit