Biphenyl dioxygenase, the first enzyme of the biphenyl catabolic pathway, is a major determinant of which polychlorinated biphenyl (PCB) congeners are metabolized by a given bacterial strain. Ongoing efforts aim to engineer BphAE, the oxygenase component of the enzyme, to efficiently transform a wider range of congeners. BphAEII9, a variant of BphAELB400 in which a seven-residue segment, 335TFNNIRI341, has been replaced by the corresponding segment of BphAEB356, 333GINTIRT339, transforms a broader range of PCB congeners than does either BphAELB400 or BphAEB356, including 2,6-dichlorobiphenyl, 3,3’-dichlorobiphenyl, 4,4’-dichlorobiphenyl, and 2,3,4’-trichlorobiphenyl. To understand the structural basis of the enhanced activity of BphAEII9, we have detd. the three-dimensional structure of this variant in substrate-free and biphenyl-bound forms. Structural comparison with BphAELB400 reveals a flexible active-site mouth and a relaxed substrate binding pocket in BphAEII9 that allow it to bind different congeners and which could be responsible for the enzyme’s altered specificity. Biochem. expts. revealed that BphAEII9 transformed 2,3,4’-trichlorobiphenyl and 2,2’,5,5’-tetrachlorobiphenyl more efficiently than did BphAELB400 and BphAEB356. BphAEII9 also transformed the insecticide dichlorodiphenyltrichloroethane (DDT) more efficiently than did either parental enzyme (apparent kcat/Km of 2.2 ± 0.5 mM-1 s-1, vs. 0.9 ± 0.5 mM-1 s-1 for BphAEB356). Studies of docking of the enzymes with these three substrates provide insight into the structural basis of the different substrate selectivities and regiospecificities of the enzymes. [on SciFinder(R)]