PCB dechlorinases revealed at last

It has taken three decades from the first report of microbial polychlorinated biphenyl (PCB) dechlorination to identify even one of the enzymes responsible. By combining conventional techniques with their own ingenuity, the latest technologies, and a bit of luck, Wang et al., in PNAS, have identified not one, but three distinct enzymes that can reductively dechlorinate PCBs (1). This finding is important because, despite being banned in the 1970s, PCBs still contaminate the sediments of rivers, lakes, and harbors worldwide. PCBs are notorious for their ability to bioaccumulate and biomagnify in the food chain, and for their multiple suspected health effects. Commercial mixtures of PCBs, known in the United States as Aroclors, are complex mixtures of 60–90 types, congeners, of PCB molecule that differ in the number (1–10) and position of chlorines on the phenyl rings. PCBs were used for decades as dielectric fluids in capacitors and transformers, and as hydraulic fluids, heat transfer fluids, lubricants and cutting oils, and additives in a variety of products (2). For example, Aroclor 1260 is a mixture of PCBs with five to eight chlorines that was used as transformer dielectric fluid. Thirty years ago it was discovered that PCBs in the anaerobic sediments of rivers were being dechlorinated by unknown agents, presumably anaerobic bacteria (3, 4). This discovery offered the best hope for an effective means of dealing with the notoriously persistent PCBs. PCB dechlorination helps to reduce the toxicity and bioaccumulation potential of PCBs and makes them more susceptible to oxidation and destruction by many organisms. However, despite years of research in multiple laboratories, the PCB-dechlorinating agents were not identified until 2007. In that year two different laboratories identified Dehalococcoides mccartyi as the bacterium responsible for dechlorinating Aroclor 1260 in aquatic …