The o-xylene dioxygenase from Rhodococcus sp. strain DK17 possesses the ability to perform distinct regioselective hydroxylations depending on the position of the substituent groups on the aromatic ring. Bioconversion experiments were performed against the non-growth substrates p-xylene, biphenyl, and naphthalene, using induced cells of Escherichia coli BL21 (DE3) harboring a recombinant expression plasmid of the DK17 o-xylene dioxygenase. Oxidation metabolites transformed from each substrate during the incubation were identified by a gas chromatography-mass spectrometry. p-Xylene was oxidized to cis-p-xylene dihydrodiol. Biphenyl and naphthalene were transformed into cis-2,3-biphenyl dihydrodiol and cis-1,2-naphthalene dihydrodiol, respectively. Considering that the DK17 o-xylene dioxygenase hydroxylates toluene and ethylbenzene at the 2,3 and the 3,4 positions on the aromatic ring in the ratios of 8:2 and 9: 1, it is apparent that the size as well as the position of the substituent groups on the aromatic ring affect the regioselectivity of aromatic oxidation by this enzyme. (c) 2007 Elsevier Inc. All rights reserved.