2,2 -Dichlorobiphenyl (CB) is transformed by the biphenyl dioxygenase of Burkholderia xenovorans LB400 (LB400 BPDO) into two metabolites (1 and 2). The most abundant metabolite, 1, was previously identified as 2,3-dihydroxy-2 -chlorobiphenyl and was presumed to originate from the initial attack by the oxygenase on the chlorine-bearing ortho carbon and on its adjacent meta carbon of one phenyl ring. 2,3,2 ,3 -Tetrachlorobiphenyl is transformed by LB400 BPDO into two metabolites that had never been fully characterized structurally. We determined the precise identity of the metabolites produced by LB400 BPDO from 2,2 -CB and 2,3,2 ,3 -CB, thus providing new insights on the mechanism by which 2,2 -CB is dehalogenated to generate 2,3-dihydroxy-2 -chlorobiphenyl. We reacted 2,2 -CB with the BPDO variant p4, which produces a larger proportion of metabolite 2. The structure of this compound was determined as cis-3,4-dihydro-3,4-dihydroxy-2,2 -dichlorobiphenyl by NMR. Metabolite 1 obtained from 2,2 -CB-d 8 was determined to be a dihydroxychlorobiphenyl-d 7 by gas chromatographic-mass spectrometric analysis, and the observed loss of only one deuterium clearly shows that the oxygenase attack occurs on carbons 2 and 3. An alternative attack at the 5 and 6 carbons followed by a rearrangement leading to the loss of the ortho chlorine would have caused the loss of more than one deuterium. The major metabolite produced from catalytic oxygenation of 2,3,2 ,3 -CB by LB400 BPDO was identified by NMR as cis-4,5-dihydro-4,5-dihydroxy-2,3,2 ,3 -tetrachlorobiphenyl. These findings show that LB400 BPDO oxygenates 2,2 -CB principally on carbons 2 and 3 and that BPDO regiospecificity toward 2,2 -CB and 2,3,2, ,3 -CB disfavors the dioxygenation of the chlorine-free orthometa carbons 5 and 6 for both congeners.The enzymes of the bacterial biphenyl catabolic pathway are very versatile. They can co-metabolically transform several polychlorinated biphenyls (1). The initial reaction of this pathway is catalyzed by the biphenyl dioxygenase (BPDO) 1 (2, 3). The cis- (2R,3S)-dihydroxy-1-phenylcyclohexa-4,6-diene (commonly called cis-2,3-dihydro-2,3-dihydroxybiphenyl) generated by the catalytic oxygenation of biphenyl is dehydrogenated by the 2,3-dihydro-2,3-dihydroxybiphenyl-2,3-dehydrogenase (BphB) and the catechol produced is cleaved by the 2,3-dihydroxybiphenyl-1,2-dioxygenase (BphC). The 2-hydroxy-6-oxo-6-phenyl-hexa-2,4-dienoic acid produced is then hydrolyzed by the 2-hydroxy-6-oxo-6-phenyl-hexa-2,4-dienoic acid hydrolase to yield benzoate and 2-hydroxypentanoate (Fig. 1). The substrate specificity of BPDO is crucial, because it limits the range of compounds that can potentially be degraded by the catabolic system. BPDO is a three-component enzymatic system (4 -6) (Fig. 1). The first component is an iron-sulfur protein (ISP BPH ) that interacts with the substrate to catalyze the addition of molecular oxygen. The second and third components are a flavoprotein reductase and a ferredoxin that are involved in the transfer of electrons from N...