Biochemistry and genetics of PCB metabolism

Biphenyl(BP)-utilizing bacteria, which include both Gram-negative and Gram-positive strains, are ubiquitously distributed in the environment. These bacteria co-metabolically degrade a variety of polychlorinated biphenyl (PCB) congeners to the corresponding chlorobenzoic acids through 2,3-dioxygenation. Certain strains degrade even highly chlorinated PCBs through 3,4-dioxygenation. The ring meta-cleavage dioxygenase purified from Pseudomonas pseudoalcaligenes KF707 is a homo-octamer containing ferrous ions as the essential cofactor. Transposon mutants revealed that the bph-encoded enzymes possess a wide range of substrate specificity for various aromatic hydrocarbons. The bphABCXD gene cluster coding for the degradation of PCBs to chlorobenzoic acids was first cloned from P. pseudocaligenes KF707 and sequenced and then was cloned from a number of BP-utilizing strains and sequenced. Some strains possess a bph operon that is very similar, if not identical, to that of KF707. Some bph genes share homologies with different degrees. Deletion and shuffling of bph genes are also found.