COMMENTARY COMMENTARY PCB dechlorinases revealed at last

Donna L. Bedard1 Department of Biological Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180

It has taken three decades from the first reductive dehalogenase enzymes (RDases) report of microbial to assist in its highly restricted way of life (PCB) dechlorination to identify even one of (7, 8). At this point, hundreds of different the enzymes responsible. By combining con- D. mccartyi RDases have been identified ventional techniques with their own ingenu- and sequenced. However, the difficulty ity, the latest technologies, and a bit of luck, of growing these organisms and low bio- Wang et al., in PNAS, have identified not mass yields have prevented researchers from one, but three distinct enzymes that can identifying the substrates of all but a handful reductively dechlorinate PCBs (1). This find- of these enzymes. Those that have been ing is important because, despite being identified include several tetrachloroethene banned in the 1970s, PCBs still contaminate (PCE) dehalogenases, a trichloroethene the sediments of rivers, lakes, and harbors (TCE) dehalogenase, two vinyl chloride worldwide. PCBs are notorious for their dehalogenases, and a chlorinated ability to bioaccumulate and biomagnify in dehalogenase (8). the food chain, and for their multiple sus- The isolation of D. mccartyi strains that pected health effects. can grow using highly chlorinated PCBs for Commercial mixtures of PCBs, known in respiration has been severely hampered by Fig. 1. Dechlorination of PCE and several PCBs by three the United States as Aroclors, are complex the inability to grow these organisms to high different PCB dechlorinases. Note the different specific- mixtures of 60–90 types, congeners, of PCB cell density because of the extreme insolubil- ities for PCB dechlorination. molecule that differ in the number (1–10) ity of PCBs. Wang et al., in Jianzhong He’s and position of chlorines on the phenyl rings. laboratory, have overcome this problem by several different PCE RDases have already PCBs were used for decades as dielectric fluids using a more soluble alternative substrate, been identified in D. mccartyi, and (ii) the in capacitors and transformers, and as hy- tetrachloroethene (PCE), to grow, isolate, PCE molecule looks nothing like a PCB draulic fluids, heat transfer fluids, lubricants and characterize the genome of three new D. mccartyi (Fig. 1). However, the experimental evidence and cutting oils, and additives in a variety of strains of that can respire the is undeniable. In each case the same RDase products (2). For example, Aroclor 1260 is highly chlorinated commercial PCB mixture gene is the most highly transcribed, whether a mixture of PCBs with five to eight chlorines Aroclor 1260 (1). It has long been known growing with PCE or PCBs, and each of the that was used as transformer dielectric fluid. that many strains of this can respire three gel-purified PCB RDases dechlorinates Thirty years ago it was discovered that PCE, and the authors reasoned that PCE both PCE and PCBs (1). PCBs in the anaerobic sediments of rivers might offer a more rapid means of growing The discovery that PCE and PCB dechlor- were being dechlorinated by unknown agents, and further enriching PCB-respiring inase capabilities are linked on a single en- presumably anaerobic bacteria (3, 4). This from cultures that had already been enriched zyme(1)hasenormousimplicationsforPCB discovery offered the best hope for an effec- with PCBs. Indeed, the authors showed that remediation. The ability to grow these PCB tive means of dealing with the notoriously the three new strains grew to a 12.5- to 22- dechlorinators with PCE as the electron ac- persistent PCBs. PCB dechlorination helps fold higher cell density in 30 days with ceptor suddenly makes the possibility