Dioxygenases catalyze the incorporation of both atoms of molecular oxygen into substrates. Cleavage of aromatic rings is one of the most important function of dioxygenases. The substrates of ring-cleavage dioxygenases can be classified into two groups according to the mode of scission of the aromatic ring. Intradiol enzymes cleave the aromatic ring between two hydroxyl groups, whereas extradiol enzymes cleave the aromatic ring between a hydroxylated carbon and another adjacent nonhydroxylated carbon. Extradiol dioxygenases are usually homomultimeric, bind one atom of ferrous ion per subunit and have a subunit size of about 33 Kd. It has been shown [1,2] that the known extradiol dioxygenases are evolutionary related. The enzymes that belong to this family are:

• Catechol 2,3-dioxygenase (EC 1.13.11.2) (metapyrocatechase) (genes nahH, xylE, dmpB, mcpII, and pheB).
• 3-methylcatechol 2,3-dioxygenase (EC 1.13.11.-) (gene todE).
• Biphenyl-2,3-diol 1,2-dioxygenase (EC 1.13.11.39) (DHBD) (gene bphC). It should be noted that in Rhodococcus globerulus, three different isozymes of DHBD have been found (genes bphC1 to bphC3). bphC1 is a classical extradiol dioxygenase, but bphC2 and bphC3 are smaller proteins (189 residues).
• 1,2-dihydroxynaphthalene dioxygenase (EC 1.13.11.-) (gene nahC).
• 2,2',3-trihydroxybiphenyl dioxygenase (EC 1.13.11.-) (gene dbfB).

As a signature pattern for these enzymes we selected a region that includes four conserved residues. Among them is a glutamate which has been shown [3], in bphC, to be implicated in the binding of the ferrous iron atom.