Transformation of RDX and other energetic compounds by xenobiotic reductases XenA and XenB

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Kinetic isotopic fractionation of carbon and nitrogen during RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) biodegradation was investigated with pure bacterial cultures under aerobic and anaerobic conditions. Relatively large bulk enrichments in 15 ؊ production were much larger than, but systematically related to, the bulk RDX N isotope effects with different bacteria. Apparent intrinsic 15 N-NO 2 ؊ values were consistent with an initial denitration pathway in the aerobic experiments and more complex processes of NO 2 ؊ formation associated with anaerobic ring cleavage. These results indicate the potential for isotopic analysis of residual RDX for the differentiation of degradation pathways and indicate that further efforts to examine the isotopic composition of potential RDX degradation products (e.g., NO x ) in the environment are warranted. IMPORTANCEThis work provides the first systematic evaluation of the isotopic fractionation of carbon and nitrogen in the organic explosive RDX during degradation by different pathways. It also provides data on the isotopic effects observed in the nitrite produced during RDX biodegradation. Both of these results could lead to better understanding of the fate of RDX in the environment and help improve monitoring and remediation technologies. Explosives manufacturing, testing, and training range activities have resulted in the release of explosive compounds, including the nitramine explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), in soils at numerous U.S. Department of Defense (DoD) installations. Because RDX is mobile in soils and persistent, it has also been detected in groundwater or drinking water at some of these sites (1, 2).There has been extensive research examining the biological degradation of explosive compounds by pure cultures of bacteria as well as mixed consortia (for a review, see reference 3). RDX biodegradation has been observed under a range of redox conditions (4-9), and at least three predominant degradation pathways have been elucidated (Fig. 1). Typically, RDX is degraded anaerobically as an electron acceptor, requiring a carbon source as an electron donor, and entails either successive reduction of the three nitro groups via successive two-electron (2e Ϫ ) transfers followed by ring cleavage (pathway A) or an earlier opening of the ring structure (pathway B1), with or without initial denitration (pathway B2). The specific mechanism of ring destabilization and the steps that lead to ring cleavage may be variable and remain subject to debate (6,7,10). A denitration reaction that converts hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX) (in pathway A) through multiple intermediates to methylenedinitramine (MEDINA) (in pathway B) under anaerobic conditions has also been proposed (7,11,12). Aerobic degradation proceeds via two single-electron transfer steps resulting in sequential denitration (2 NO 2 Ϫ removed) followed by ring cleavage and production of 4-nitro-2,4-diazabutanal (NDAB) (pathway C) (7). Some bacteria are able to utilize R...

Section: Discussionmentioning

confidence: 99%

“…RDX biodegradation has been observed under a range of redox conditions (4)(5)(6)(7)(8)(9), and at least three predominant degradation pathways have been elucidated (Fig. 1).…”

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confidence: 99%

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confidence: 99%

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Relating Carbon and Nitrogen Isotope Effects to Reaction Mechanisms during Aerobic or Anaerobic Degradation of RDX (Hexahydro-1,3,5-Trinitro-1,3,5-Triazine) by Pure Bacterial Cultures

Fuller

Heraty

Condee

et al. 2016

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“…In situ bioremediation offers an attractive, cost-effective cleanup option for contaminated sites with limited access. Microbially mediated RDX biodegradation has been reported under a number of conditions (5)(6)(7)(8)(9)(10)(11)(12)(13)(14), with only a few genes implicated in RDX degradation (15)(16)(17). Of these, the cytochrome P450 system encoded by xplAB is the best characterized (18,19).…”

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confidence: 99%

Role of Nitrogen Limitation in Transformation of RDX (Hexahydro-1,3,5-Trinitro-1,3,5-Triazine) by Gordonia sp. Strain KTR9

Indest

Hancock

Jung

et al. 2013

“…Mobile explosive contaminants, like RDX, threaten overall sustainability of Department of Defense (DoD) ranges by migrating away from the source areas and contaminating surface and subsurface soils at relatively low but potentially toxic levels. The potential for bioremediation of RDX in groundwater and soils via the addition of nutrients (19,23) or via combined abiotic/biotic processes (49) has been established. While in situ bioremediation is an attractive remedial action for these sites, the capability to predict the efficacy of in situ bioremediation of RDX is generally lacking.…”

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confidence: 99%

Functional Characterization of pGKT2, a 182-Kilobase Plasmid Containing the xplAB Genes, Which Are Involved in the Degradation of Hexahydro-1,3,5-Trinitro-1,3,5-Triazine by Gordonia sp. Strain KTR9

Indest

Jung

Chen

et al. 2010

Several microorganisms have been isolated that can transform hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), a cyclic nitramine explosive. To better characterize the microbial genes that facilitate this transformation, we sequenced and annotated a 182-kb plasmid, pGKT2, from the RDX-degrading strain Gordonia sp. KTR9. This plasmid carries xplA, encoding a protein sharing up to 99% amino acid sequence identity with characterized RDX-degrading cytochromes P450. Other genes that cluster with xplA are predicted to encode a glutamine synthase-XplB fusion protein, a second cytochrome P450, Cyp151C, and XplR, a GntR-type regulator. Rhodococcus jostii RHA1 expressing xplA from KTR9 degraded RDX but did not utilize RDX as a nitrogen source. Moreover, an Escherichia coli strain producing XplA degraded RDX but a strain producing Cyp151C did not. KTR9 strains cured of pGKT2 did not transform RDX. Physiological studies examining the effects of exogenous nitrogen sources on RDX degradation in strain KTR9 revealed that ammonium, nitrite, and nitrate each inhibited RDX degradation by up to 79%. Quantitative real-time PCR analysis of glnA-xplB, xplA, and xplR showed that transcript levels were 3.7-fold higher during growth on RDX than during growth on ammonium and that this upregulation was repressed in the presence of various inorganic nitrogen sources. Overall, the results indicate that RDX degradation by KTR9 is integrated with central nitrogen metabolism and that the uptake of RDX by bacterial cells does not require a dedicated transporter.