Genetically modified organisms to remediate polychlorinated biphenyls. Where do we stand?

The biphenyl catabolic pathway transforms selected polychlorinated biphenyls (PCBs) to chlorobenzoates. The inability of bacteria to degrade the persistent PCBs is due to failure of enzymes of this pathway to catalyze their transformation. Biphenyl 2,3-dioxygenase (BPDO), 2,3-dihydroxybiphenyl 1,2-dioxygenase (HPDO) and 2-hydroxy-6-oxo-6-phenyl-2,4-hexadienoate (HOPDA) hydrolase (HOPDAH) are critical to determine the range of PCBs degraded. Investigations are under way to examine features of these enzymes that influence their interactions with chlorinated substrates and to propose ways to bypass the blockages that they cause. BPDO terminal oxygenase component is an hexamer comprised of an α and a β subunit. Burkholderia sp. LB400 and Pseudomonas pseudoalcaligenes KF707 BPDOs are very similar but exhibit quite distinct substrate specificities. Comamonas testosteroni B-356 and Rhodoccocus globerulus P6 BPDOs are more distantly related to LB400 BPDO and show a different range of PCB substrate used. Comparative analyses of the catalytic properties of chimeras of these enzymes unrevealed several regions of the C-terminal domain of the α subunit that strongly influence BPDOʹs substrate specificity. DNA shuffling between genes encoding these homologous BPDOs has been used to broaden the enzymeʹs substrate specificity. HPDOs are unable to oxygenate 3,4-dihydroxylated metabolites of chlorobiphenyls. HOPDAH are limited in their capacity to hydrolyze HOPDAs bearing chloro substituents on the dienoate moiety. Enzymes homologous to HPDO and HOPDAH but exhibiting different patterns of substrate specificity have been identified. They provide tools to examine the enzymes structural features responsible for substrate specificity and to plan strategies to extend the range of chlorinated substrates that they can transform.