Dehalogenation of Polybrominated Diphenyl Ethers and Polychlorinated Biphenyls Catalyzed by a Reductive Dehalogenase in Dehalococcoides mccartyi Strain MB

Polybrominated diphenyl ethers (PBDEs)
and polychlorinated biphenyls
(PCBs) are notorious persistent organic pollutants. However, few organohalide-respiring
bacteria that harbor reductive dehalogenases (RDases) capable of dehalogenating
these pollutants have been identified. Here, we report reductive dehalogenation
of penta-BDEs and PCBs byDehalococcoides mccartyi strain MB. The PCE-pregrown cultures of strain MB debrominated 86.6
± 7.4% penta-BDEs to di- to tetra-BDEs within 5 days. Similarly,
extensive dechlorination of Aroclor1260 and Aroclor1254 was observed
in the PCE-pregrown cultures of strain MB, with the average chlorine
per PCB decreasing from 6.40 ± 0.02 and 5.40 ± 0.03 to 5.98
± 0.11 and 5.19 ± 0.07 within 14 days, respectively; para-substituents were preferentially dechlorinated from
PCBs. Moreover, strain MB showed distinct enantioselective dechlorination
of different chiral PCB congeners. Dehalogenation activity and cell
growth were maintained during the successive transfer of cultures
when amended with penta-BDEs as the sole electron acceptors but not
when amended with only PCBs, suggesting metabolic and co-metabolic
dehalogenation of these compounds, respectively. Transcriptional analysis,
proteomic profiling, and in vitro activity assays
indicated that MbrA was involved in dehalogenating PCE, PCBs, and
PBDEs. Interestingly, resequencing of mbrA in strain
MB identified three nonsynonymous mutations within the nucleotide
sequence, although the consequences of which remain unknown. The substrate
versatility of MbrA enabled strain MB to dechlorinate PCBs in the
presence of either penta-BDEs or PCE, suggesting that co-metabolic
dehalogenation initiated by multifunctional RDases may contribute
to PCB attenuation at sites contaminated with multiple organohalide
pollutants.