Two different types of dehalogenases, LinA and LinB, involved in gamma-hexachlorocyclohexane degradation in Sphingomonas paucimobilis UT26 are localized in the periplasmic space without molecular processing

Transgenic Arabidopsis thaliana plants expressing bacterial γ-hexachlorocyclohexane dehydrochlorinase LinA

BACKGROUND

γ-Hexachlorocyclohexane (γ-HCH), an organochlorine insecticide of anthropogenic origin, is a persistent organic pollutant (POP) that causes environmental pollution concerns worldwide. Although many γ-HCH-degrading bacterial strains are available, inoculating them directly into γ-HCH-contaminated soil is ineffective because of the low survival rate of the exogenous bacteria. Another strategy for the bioremediation of γ-HCH involves the use of transgenic plants expressing bacterial enzyme for γ-HCH degradation through phytoremediation.

RESULTS

We generated transgenic Arabidopsis thaliana expressing γ-HCH dehydrochlroninase LinA from bacterium Sphingobium japonicum strain UT26. Among the transgenic Arabidopsis T2 lines, we obtained one line (A5) that expressed and accumulated LinA well. The A5-derived T3 plants showed higher tolerance to γ-HCH than the non-transformant control plants, indicating that γ-HCH is toxic for Arabidopsis thaliana and that this effect is relieved by LinA expression. The crude extract of the A5 plants showed γ-HCH degradation activity, and metabolites of γ-HCH produced by the LinA reaction were detected in the assay solution, indicating that the A5 plants accumulated the active LinA protein. In some A5 lines, the whole plant absorbed and degraded more than 99% of γ-HCH (10 ppm) in the liquid medium within 36 h.

CONCLUSION

The transgenic Arabidopsis expressing active LinA absorbed and degraded γ-HCH in the liquid medium, indicating the high potential of LinA-expressing transgenic plants for the phytoremediation of environmental γ-HCH. This study marks a crucial step toward the practical use of transgenic plants for the phytoremediation of POPs.

Occurrence and diffusive air-seawater exchanges of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in Fildes Bay, King George Island, Antarctica

We report the levels of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in seawater and air, and the air-sea dynamics through diffusive exchange analysis in Fildes Bay, King George Island, Antarctica, between November 2019 and January 30, 2020. Hexachlorobenzene (HCB) was the most abundant compound in both air and seawater with concentrations around 39 ± 2.1 pg m-3 and 3.2 ± 2.4 pg L-1 respectively. The most abundant PCB congener was PCB 11, with a mean of 3.16 ± 3.7 pg m-3 in air and 2.0 ± 1.1 pg L-1 in seawater. The fugacity gradient estimated for the OCP compounds indicate a predominance of net atmospheric deposition for HCB, α-HCH, γ-HCH, 4,4'-DDT, 4,4'-DDE and close to equilibrium for the PeCB compound. The observed deposition of some OCs may be driven by high biodegradation rates and/or settling fluxes decreasing the concentration of these compounds in surface waters, which is supported by the capacity of microbial consortium to degrade some of these compounds. The estimated fugacity gradients for PCBs showed differences between congeners, with net volatilization predominating for PCB-9, a trend close to equilibrium for PCB congeners 11, 28, 52, 101, 118, 138, and 153, and deposition for PCB 180. Snow amplification may play an important role for less hydrophobic PCBs, with volatilization predominating after snow/glacier melting. As hydrophobicity increases, the biological pump decreases the concentration of PCBs in seawater, reversing the fugacity gradient to atmospheric deposition. This study highlights the potential impacts of climate change, through glacier retreat, on the biogeochemistry of POPs, remobilizing those compounds previously trapped within the cryosphere which in turn will transform the Antarctic cryosphere into a secondary source of the more volatile POPs in coastal areas, influenced by snow and ice melting.

Degradation of DDT by γ-hexachlorocyclohexane dehydrochlorinase LinA

1,1,1-Trichloro-2,2-bis(4-chlorophenyl)-ethane (DDT) is the first synthetic insecticide and one of the most widely used pesticides. The use of DDT has been banned, but it remains one of the most notorious environmental pollutants around the world. In this study, we found that γ-hexachlorocyclohexane (γ-HCH) dehydrochlorinase LinA from a γ-HCH-degrading bacterium, Sphingobium japonicum UT26, converts DDT to 1,1-dichloro-2,2-bis(4-chlorophenyl)-ethylene (DDE). Because of the weak DDT degradation activity of LinA, we could not detect such activity in UT26 cells expressing LinA constitutively. However, the linA-deletion mutant of UT26 harboring a plasmid for the expression of LinA, in which LinA was expressed at a higher level than UT26, showed the DDT degradation activity. This outcome highlights the potential for constructing DDT-degrading sphingomonad cells through elevated LinA expression.

Enhanced biodegradation of hexachlorocyclohexane (HCH) isomers by Sphingobium sp. strain D4 in the presence of root exudates or in co-culture with HCH-mobilizing strains

Lindane and other 1,2,3,4,5,6-hexachlorocyclohexane (HCH) isomers are persistent organic pollutants highly hydrophobic, which hampers their availability and biodegradation. This work aimed at (i) investigating genes encoding enzymes involved in HCH degradation in the bacterium Sphingobium sp. D4, (ii) selecting strains, from a collection of environmental isolates, able to mobilize HCHs from contaminated soil, and (iii) analysing the biodegradation of HCHs by strain D4 in co-culture with HCH-mobilizing strains or when cultivated with root exudates. Fragments of the same size and similar sequence to linA and linB genes were successfully amplified. Two isolates, Streptomyces sp. M7 and Rhodococcus erythropolis ET54b able to produce emulsifiers and to mobilize HCH isomers from soil were selected. Biodegradation of HCH isomers by strain D4 was enhanced when co-inoculated with HCH mobilizing strains or when cultivated with root exudates. The degrader strain D4 was able to decompose very efficiently HCHs isomers, reducing their concentration in soil slurries by more than 95% (from an average initial amount of 50 ± 8 mg HCH kg^-1 soil) in 9 days. The combination of HCH-degrading and HCH-mobilizing strains can be considered a promising inoculum for future soil bioremediation studies using bioaugmentation techniques or in combination with plants in rhizodegradation assays.

Bioremediation of lindane contaminated soil: Exploring the potential of actinobacterial strains

Lindane, an organochlorine pesticide, causes detrimental impacts on the environment and human health owing to its high toxicity, low degradation, and bioaccumulation. Its toxic nature can be overcome by biological and eco-friendly approaches involving its degradation and detoxification. The biodegradation of lindane was assessed using actinobacterial species Thermobifida cellulosilytica TB100 (T. cellulosilytica), Thermobifida halotolerans DSM 44931 (T. halotolerans) and Streptomyces coelicolor A3 (S. coelicolor). The degradation conditions of Lindane such as pH, temperature, inoculum volume, glucose concentration and number of days were optimized under broth conditions. Lindane degradation at different concentrations was studied in soil using reverse phase-high performance liquid chromatography over a 30 day period. A bioassay test was performed on seeds of Lactuca sativa (Lettuce) to assess the success of bioremediated soil. Maximum lindane degradation in soil was observed using T. cellulosilytica sp. The degradation trend for different concentrations of lindane using T. halotolerans in sterilized soil was 55 mg kg^-1 (82%) ˃ 155 mg kg^-1 (75%) ˃ 255 mg kg^-1 (70%) after an incubation period of 30 days. Lindane degradation in soil followed the first order reaction kinetics. Phytotoxicity test on seeds of Lactuca sativa showed considerably good vigor index values for the bioremediated sterilized and non-sterilized soil by T. cellulosilytica, T. halotolerans and S. coelicolor in comparison to the contaminated soil without bacteria. This confirms that these actinobacterial species can be implemented in bioaugmentation of contaminated sites to efficiently remediate high lindane concentrations.

Expression of an alcohol dehydrogenase gene in a heterotrophic bacterium induces carbon dioxide-dependent high-yield growth under oligotrophic conditions

Sphingobium japonicum strain UT26, whose γ-hexachlorocyclohexane-degrading ability has been studied in detail, is a typical aerobic and heterotrophic bacterium that needs organic carbon sources for its growth, and cannot grow on a minimal salt agar medium prepared without adding any organic carbon sources. Here, we isolated a mutant of UT26 with the ability to grow to visible state on such an oligotrophic medium from a transposon-induced mutant library. This high-yield growth under oligotrophic conditions (HYGO) phenotype was CO₂-dependent and accompanied with CO₂ incorporation. In the HYGO mutant, a transposon was inserted just upstream of the putative Zn-dependent alcohol dehydrogenase (ADH) gene (adhX) so that the adhX gene was constitutively expressed, probably by the transposon-derived promoter. The adhX-deletion mutant (UT26DAX) harbouring a plasmid carrying the adhX gene under the control of a constitutive promoter exhibited the HYGO phenotype. Moreover, the HYGO mutants spontaneously emerged among the UT26-derived hypermutator strain cells, and adhX was highly expressed in these HYGO mutants, while no HYGO mutant appeared among UT26DAX-derived hypermutator strain cells, indicating the necessity of adhX for the HYGO phenotype. His-tagged AdhX that was expressed in Escherichia coli and purified to homogeneity showed ADH activity towards methanol and other alcohols. Mutagenesis analysis of the adhX gene indicated a correlation between the ADH activity and the HYGO phenotype. These results demonstrated that the constitutive expression of an adhX-encoding protein with ADH activity in UT26 leads to the CO₂-dependent HYGO phenotype. Identical or nearly identical adhX orthologues were found in other sphingomonad strains, and most of them were located on plasmids, suggesting that the adhX-mediated HYGO phenotype may be an important adaptation strategy to oligotrophic environments among sphingomonads.

Degradation strategies and associated regulatory mechanisms/features for aromatic compound metabolism in bacteria

As a result of anthropogenic activity, large number of recalcitrant aromatic compounds have been released into the environment. Consequently, microbial communities have adapted and evolved to utilize these compounds as sole carbon source, under both aerobic and anaerobic conditions. The constitutive expression of enzymes necessary for metabolism imposes a heavy energy load on the microbe which is overcome by arrangement of degradative genes as operons which are induced by specific inducers. The segmentation of pathways into upper, middle and/or lower operons has allowed microbes to funnel multiple compounds into common key aromatic intermediates which are further metabolized through central carbon pathway. Various proteins belonging to diverse families have evolved to regulate the transcription of individual operons participating in aromatic catabolism. These proteins, complemented with global regulatory mechanisms, carry out the regulation of aromatic compound metabolic pathways in a concerted manner. Additionally, characteristics like chemotaxis, preferential utilization, pathway compartmentalization and biosurfactant production confer an advantage to the microbe, thus making bioremediation of the aromatic pollutants more efficient and effective.

Genomic Analysis of γ-Hexachlorocyclohexane-Degrading Sphingopyxis lindanitolerans WS5A3p Strain in the Context of the Pangenome of Sphingopyxis

Sphingopyxis inhabit diverse environmental niches, including marine, freshwater, oceans, soil and anthropogenic sites. The genus includes 20 phylogenetically distinct, valid species, but only a few with a sequenced genome. In this work, we analyzed the nearly complete genome of the newly described species, Sphingopyxislindanitolerans, and compared it to the other available Sphingopyxis genomes. The genome included 4.3 Mbp in total and consists of a circular chromosome, and two putative plasmids. Among the identified set of lin genes responsible for γ-hexachlorocyclohexane pesticide degradation, we discovered a gene coding for a new isoform of the LinA protein. The significant potential of this species in the remediation of contaminated soil is also correlated with the fact that its genome encodes a higher number of enzymes potentially involved in aromatic compound degradation than for most other Sphingopyxis strains. Additional analysis of 44 Sphingopyxis representatives provides insights into the pangenome of Sphingopyxis and revealed a core of 734 protein clusters and between four and 1667 unique proteins per genome.

Compartmentalization of the Carbaryl Degradation Pathway: Molecular Characterization of Inducible Periplasmic Carbaryl Hydrolase from Pseudomonas spp

Pseudomonas sp. strains C5pp and C7 degrade carbaryl as the sole carbon source. Carbaryl hydrolase (CH) catalyzes the hydrolysis of carbaryl to 1-naphthol and methylamine. Bioinformatic analysis of mcbA, encoding CH, in C5pp predicted it to have a transmembrane domain (Tmd) and a signal peptide (Sp). In these isolates, the activity of CH was found to be 4- to 6-fold higher in the periplasm than in the cytoplasm. The recombinant CH (rCH) showed 4-fold-higher activity in the periplasm of Escherichia coli The deletion of Tmd showed activity in the cytoplasmic fraction, while deletion of both Tmd and Sp (Tmd+Sp) resulted in expression of the inactive protein. Confocal microscopic analysis of E. coli expressing a (Tmd+Sp)-green fluorescent protein (GFP) fusion protein revealed the localization of GFP into the periplasm. Altogether, these results indicate that Tmd probably helps in anchoring of polypeptide to the inner membrane, while Sp assists folding and release of CH in the periplasm. The N-terminal sequence of the mature periplasmic CH confirms the absence of the Tmd+Sp region and confirms the signal peptidase cleavage site as Ala-Leu-Ala. CH purified from strains C5pp, C7, and rCHΔ(Tmd)a were found to be monomeric with molecular mass of ∼68 to 76 kDa and to catalyze hydrolysis of the ester bond with an apparent K_m and V_max in the range of 98 to 111 μM and 69 to 73 μmol · min^-1 · mg^-1, respectively. The presence of low-affinity CH in the periplasm and 1-naphthol-metabolizing enzymes in the cytoplasm of Pseudomonas spp. suggests the compartmentalization of the metabolic pathway as a strategy for efficient degradation of carbaryl at higher concentrations without cellular toxicity of 1-naphthol.IMPORTANCE Proteins in the periplasmic space of bacteria play an important role in various cellular processes, such as solute transport, nutrient binding, antibiotic resistance, substrate hydrolysis, and detoxification of xenobiotics. Carbaryl is one of the most widely used carbamate pesticides. Carbaryl hydrolase (CH), the first enzyme of the degradation pathway which converts carbaryl to 1-naphthol, was found to be localized in the periplasm of Pseudomonas spp. Predicted transmembrane domain and signal peptide sequences of Pseudomonas were found to be functional in Escherichia coli and to translocate CH and GFP into the periplasm. The localization of low-affinity CH into the periplasm indicates controlled formation of toxic and recalcitrant 1-naphthol, thus minimizing its accumulation and interaction with various cellular components and thereby reducing the cellular toxicity. This study highlights the significance of compartmentalization of metabolic pathway enzymes for efficient removal of toxic compounds.

Synergistic rhizosphere degradation of γ-hexachlorocyclohexane (lindane) through the combinatorial plant-fungal action

Fungi are usually involved in degradation/deterioration of many anthropogenic wastes due to their verse enzyme secretions and adaptive capabilities. In this study, five dominant fungal strains were isolated from an aged lindane polluted site, they were all mixed (100 mg each) together with pent mushroom compost (SMC) and applied to lindane polluted soil (5 kg) at 10, 20, 30, 40% and control 0% (soil with no treatment), these were used to grow M. maximus Jacq for 3 months. To establish lindane degradation, deductions such as Degradation rate (K1), Half-life (t1/2) and Degradation efficiency (DE) were made based on the analyzed lindane concentrations before and after the experiment. We also tested the presence and expressions of phosphoesterases (mpd and opd-A) and catechol 1,2-dioxygenases (efk2 and efk4) genes in the strains. The stains were identified as Aspergillus niger (KY693970); Talaromyces atroroseus (KY488464), Talaromyces purpurogenus (KY488468), Yarrowia lipolytica (KY488469) and Aspergillus flavus (KY693973) through morphological and molecular methods. Combined rhizospheric action of M. maximus and fungi speed up lindane degradation rate, initially detected lindane concentration of 45 mg/kg was reduced to 11.26, 9.34 and 11.23 mg/kg in 20, 30 and 40% treatments respectively making 79.76, 85.93 and 88.67% degradation efficiencies. K1 of 1.29 was recorded in control while higher K1 of 1.60, 1.96 and 2.18 /day were recorded in 20, 30 and 40% treatments respectively. The best t1/2 of 0.32 and 0.35 /day were recorded in 40 and 30% compared to control (0.54 /day). All the strains were also affirmed to possess the tested genes; opd was overexpressed in all the strains except KY693973 while mpd was overexpressed in KY693970, KY488464 but moderately expressed in KY488468, KY488469 and KY693973. However, efk genes were under-expressed in most of the strains except KY488469 and KY693973 which showed moderate expression of efk4. This work suggests that the synergistic association of the identified rhizospheric fungi and M. maximus roots could be used to remove lindane in soil at a limited time period and this combination could be used at large scale.

Recent strategies for removal and degradation of persistent & toxic organochlorine pesticides using nanoparticles: A review

Organochlorines (OCs) are the most hazardous class of pesticides, therefore, banned or restricted in several countries. The major sources of OCs include food industries, agriculture and sewage wastes. Their effluents discharged into the water bodies contain extremely high concentration of OCs which ultimately causes environmental concern. Because of their high persistence, toxicity and potential to bioaccumulation, their removal from wastewater is imperative. The degradation techniques are now advanced using nanomaterials of various kinds. During the last few years, nanoparticles such as TiO₂ and Fe are found to be excellent adsorbents and efficient photocatalysts for degrading more or less whole OCs as well as their toxic metabolites, which opens the opportunities for exploring various other nanoparticles as well. It is noteworthy that such methodologies are economic, fast and very efficient. In this review, the detailed information on different types of OC pesticides, their metabolites, environmental concern and present status on degradation methods using nanoparticles have been reviewed. An attempt has also been made to highlight the research gaps prevailing in the current research area.

Microbial Degradation of a Recalcitrant Pesticide: Chlordecone

Chlordecone (Kepone®) is a synthetic organochlorine insecticide (C₁₀Cl₁₀O) used worldwide mostly during the 1970 and 1980s. Its intensive application in the French West Indies to control the banana black weevil Cosmopolites sordidus led to a massive environmental pollution. Persistence of chlordecone in soils and water for numerous decades even centuries causes global public health and socio-economic concerns. In order to investigate the biodegradability of chlordecone, microbial enrichment cultures from soils contaminated by chlordecone or other organochlorines and from sludge of a wastewater treatment plant have been conducted. Different experimental procedures including original microcosms were carried out anaerobically over long periods of time. GC-MS monitoring resulted in the detection of chlorinated derivatives in several cultures, consistent with chlordecone biotransformation. More interestingly, disappearance of chlordecone (50 μg/mL) in two bacterial consortia was concomitant with the accumulation of a major metabolite of formula C₉Cl₅H₃ (named B1) as well as two minor metabolites C₁₀Cl₉HO (named A1) and C₉Cl₄H₄ (named B3). Finally, we report the isolation and the complete genomic sequences of two new Citrobacter isolates, closely related to Citrobacter amalonaticus, and that were capable of reproducing chlordecone transformation. Further characterization of these Citrobacter strains should yield deeper insights into the mechanisms involved in this transformation process.

Biodegradation of γ-hexachlorocyclohexane by transgenic hairy root cultures of Cucurbita moschata that accumulate recombinant bacterial LinA

γ-HCH was successfully degraded using LinA-expressed transgenic hairy root cultures of Cucurbita moschata . Fusing an endoplasmic reticulum-targeting signal peptide to LinA was essential for stable accumulation in the hairy roots. The pesticide γ-hexachlorocyclohexane (γ-HCH) is a persistent organic pollutant (POP) that raises public health and environmental pollution concerns worldwide. Although several isolates of γ-HCH-degrading bacteria are available, inoculating them directly into γ-HCH-contaminated soil is ineffective because of the bacterial survival rate. Cucurbita species incorporate significant amounts of POPs from soils compared with other plant species. Here, we describe a novel bioremediation strategy that combines the bacterial degradation of γ-HCH and the efficient uptake of γ-HCH by Cucurbita species. We produced transgenic hairy root cultures of Cucurbita moschata that expressed recombinant bacterial linA, isolated from the bacterium Sphingobium japonicum UT26. The LinA protein was accumulated stably in the hairy root cultures by fusing an endoplasmic reticulum (ER)-targeting signal peptide to LinA. Then, we demonstrated that the cultures degraded more than 90 % of γ-HCH (1 ppm) overnight and produced the γ-HCH metabolite 1,2,4-trichlorobenzene, indicating that LinA degraded γ-HCH. These results indicate that the gene linA has high potential for phytoremediation of environmental γ-HCH.

Comparison of the complete genome sequences of four γ-hexachlorocyclohexane-degrading bacterial strains: insights into the evolution of bacteria able to degrade a recalcitrant man-made pesticide

γ-Hexachlorocyclohexane (γ-HCH) is a recalcitrant man-made chlorinated pesticide. Here, the complete genome sequences of four γ-HCH-degrading sphingomonad strains, which are most unlikely to have been derived from one ancestral γ-HCH degrader, were compared. Together with several experimental data, we showed that (i) all the four strains carry almost identical linA to linE genes for the conversion of γ-HCH to maleylacetate (designated “specific” lin genes), (ii) considerably different genes are used for the metabolism of maleylacetate in one of the four strains, and (iii) the linKLMN genes for the putative ABC transporter necessary for γ-HCH utilization exhibit structural divergence, which reflects the phylogenetic relationship of their hosts. Replicon organization and location of the lin genes in the four genomes are significantly different with one another, and that most of the specific lin genes are located on multiple sphingomonad-unique plasmids. Copies of IS6100, the most abundant insertion sequence in the four strains, are often located in close proximity to the specific lin genes. Analysis of the footprints of target duplication upon IS6100 transposition and the experimental detection of IS6100 transposition strongly suggested that the IS6100 transposition has caused dynamic genome rearrangements and the diversification of lin-flanking regions in the four strains.

Membrane Proteins Are Dramatically Less Conserved than Water-Soluble Proteins across the Tree of Life

Membrane proteins are crucial in transport, signaling, bioenergetics, catalysis, and as drug targets. Here, we show that membrane proteins have dramatically fewer detectable orthologs than water-soluble proteins, less than half in most species analyzed. This sparse distribution could reflect rapid divergence or gene loss. We find that both mechanisms operate. First, membrane proteins evolve faster than water-soluble proteins, particularly in their exterior-facing portions. Second, we demonstrate that predicted ancestral membrane proteins are preferentially lost compared with water-soluble proteins in closely related species of archaea and bacteria. These patterns are consistent across the whole tree of life, and in each of the three domains of archaea, bacteria, and eukaryotes. Our findings point to a fundamental evolutionary principle: membrane proteins evolve faster due to stronger adaptive selection in changing environments, whereas cytosolic proteins are under more stringent purifying selection in the homeostatic interior of the cell. This effect should be strongest in prokaryotes, weaker in unicellular eukaryotes (with intracellular membranes), and weakest in multicellular eukaryotes (with extracellular homeostasis). We demonstrate that this is indeed the case. Similarly, we show that extracellular water-soluble proteins exhibit an even stronger pattern of low homology than membrane proteins. These striking differences in conservation of membrane proteins versus water-soluble proteins have important implications for evolution and medicine.

Proteomics of Sphingobium indicum B90A for a deeper understanding of hexachlorocyclohexane (HCH) bioremediation

Genome wide expression profiling of Sphingobium indicum B90A revealed induction of lin genes, linA and linB, involved in dechlorination of hexachlorocyclohexane (HCH), in the presence of all four isomers of HCH. Supporting proteomics data, the qPCR and promoter assay showed upregulation of linA transcription in the presence of HCH isomers. Analysis of the upstream region of the linA gene revealed the existence of the GntR binding site overlapping the -10 hexamer of the putative promoter motif. As GntR is a known transcription repressor its dissociation from the linA promoter is expected to induce lin genes in the presence of HCH isomers. Comparison of in situ and in-culture proteomics indicated expression lin genes at the dumpsite, an indication for the in situ HCH degradation.

Growth Inhibition by Metabolites of γ-Hexachlorocyclohexane inSphingobium japonicumUT26

The growth of a gamma-hexachlorocyclohexane (gamma-HCH)-degrading bacterium Sphingobium japonicum (formerly Sphingomonas paucimobilis) UT26 in rich medium was inhibited by gamma-HCH. This growth inhibition was not observed in a mutant that lacked the initial or second step enzymatic activity for gamma-HCH degradation, suggesting that metabolites of gamma-HCH are toxic to UT26. Two metabolites of gamma-HCH, 2,5-dichlorophenol (2,5-DCP) and 2,5-dichlorohydroquinone (2,5-DCHQ), showed a direct toxic effect on UT26 and other sphingomonad strains. Because only 2,5-DCP accumulated during gamma-HCH degradation, 2,5-DCP is thought to be a main compound for growth inhibition.

Enantioselective carbon stable isotope fractionation of hexachlorocyclohexane during aerobic biodegradation by Sphingobium spp

Carbon isotope fractionation was investigated for the biotransformation of γ- and α- hexachlorocyclohexane (HCH) as well as enantiomers of α-HCH using two aerobic bacterial strains: Sphingobium indicum strain B90A and Sphingobium japonicum strain UT26. Carbon isotope enrichment factors (ε(c)) for γ-HCH (ε(c) = -1.5 ± 0.1 ‰ and -1.7 ± 0.2 ‰) and α-HCH (ε(c) = -1.0 ± 0.2 ‰ and -1.6 ± 0.3 ‰) were similar for both aerobic strains, but lower in comparison with previously reported values for anaerobic γ- and α-HCH degradation. Isotope fractionation of α-HCH enantiomers was higher for (+) α-HCH (ε(c) = -2.4 ± 0.8 ‰ and -3.3 ± 0.8 ‰) in comparison to (-) α-HCH (ε(c) = -0.7 ± 0.2 ‰ and -1.0 ± 0.6 ‰). The microbial fractionation between the α-HCH enantiomers was quantified by the Rayleigh equation and enantiomeric fractionation factors (ε(e)) for S. indicum strain B90A and S. japonicum strain UT26 were -42 ± 16% and -22 ± 6%, respectively. The extent and range of isomer and enantiomeric carbon isotope fractionation of HCHs with Sphingobium spp. suggests that aerobic biodegradation of HCHs can be monitored in situ by compound-specific stable isotope analysis (CSIA) and enantiomer-specific isotope analysis (ESIA). In addition, enantiomeric fractionation has the potential as a complementary approach to CSIA and ESIA for assessing the biodegradation of α-HCH at contaminated field sites.

Lindane removal by pure and mixed cultures of immobilized actinobacteria

Lindane (γ-HCH) is an organochlorine insecticide that has been widely used in developing countries. It is known to persist in the environment and can cause serious health problems. One of the strategies adopted to remove lindane from the environment is bioremediation using microorganisms. Immobilized cells present advantages over free suspended cells, like their high degradation efficiency and protection against toxins. The aims of this work were: (1) To evaluate the ability of Streptomyces strains immobilized in four different matrices to remove lindane, (2) To select the support with optimum lindane removal by pure cultures, (3) To assay the selected support with consortia and (4) To evaluate the reusability of the immobilized cells. Four Streptomyces sp. strains had previously shown their ability to grow in the presence of lindane. Lindane removal by microorganisms immobilized was significantly higher than in free cells. Specifically immobilized cells in cloth sachets showed an improvement of around 25% in lindane removal compared to the abiotic control. Three strains showed significantly higher microbial growth when they were entrapped in silicone tubes. Strains immobilized in PVA-alginate demonstrated lowest growth. Mixed cultures immobilized inside cloth sachets showed no significant enhancement compared to pure cultures, reaching a maximum removal of 81% after 96 h for consortium I, consisting of the four immobilized strains together. Nevertheless, the cells could be reused for two additional cycles of 96 h each, obtaining a maximum removal efficiency of 71.5% when each of the four strains was immobilized in a separate bag (consortium III).

The evolution of new enzyme function: lessons from xenobiotic metabolizing bacteria versus insecticide-resistant insects

Here, we compare the evolutionary routes by which bacteria and insects have evolved enzymatic processes for the degradation of four classes of synthetic chemical insecticide. For insects, the selective advantage of such degradative activities is survival on exposure to the insecticide, whereas for the bacteria the advantage is simply a matter of access to additional sources of nutrients. Nevertheless, bacteria have evolved highly efficient enzymes from a wide variety of enzyme families, whereas insects have relied upon generalist esterase-, cytochrome P450- and glutathione-S-transferase-dependent detoxification systems. Moreover, the mutant insect enzymes are less efficient kinetically and less diverged in sequence from their putative ancestors than their bacterial counterparts. This presumably reflects several advantages that bacteria have over insects in the acquisition of new enzymatic functions, such as a broad biochemical repertoire from which new functions can be evolved, large population sizes, high effective mutation rates, very short generation times and access to genetic diversity through horizontal gene transfer. Both the insect and bacterial systems support recent theory proposing that new biochemical functions often evolve from 'promiscuous' activities in existing enzymes, with subsequent mutations then enhancing those activities. Study of the insect enzymes will help in resistance management, while the bacterial enzymes are potential bioremediants of insecticide residues in a range of contaminated environments.

Biochemistry of microbial degradation of hexachlorocyclohexane and prospects for bioremediation

Lindane, the gamma-isomer of hexachlorocyclohexane (HCH), is a potent insecticide. Purified lindane or unpurified mixtures of this and alpha-, beta-, and delta-isomers of HCH were widely used as commercial insecticides in the last half of the 20th century. Large dumps of unused HCH isomers now constitute a major hazard because of their long residence times in soil and high nontarget toxicities. The major pathway for the aerobic degradation of HCH isomers in soil is the Lin pathway, and variants of this pathway will degrade all four of the HCH isomers although only slowly. Sequence differences in the primary LinA and LinB enzymes in the pathway play a key role in determining their ability to degrade the different isomers. LinA is a dehydrochlorinase, but little is known of its biochemistry. LinB is a hydrolytic dechlorinase that has been heterologously expressed and crystallized, and there is some understanding of the sequence-structure-function relationships underlying its substrate specificity and kinetics, although there are also some significant anomalies. The kinetics of some LinB variants are reported to be slow even for their preferred isomers. It is important to develop a better understanding of the biochemistries of the LinA and LinB variants and to use that knowledge to build better variants, because field trials of some bioremediation strategies based on the Lin pathway have yielded promising results but would not yet achieve economic levels of remediation.

Crystallization and preliminary X-ray analysis of gamma-hexachlorocyclohexane dehydrochlorinase LinA from Sphingobium japonicum UT26

LinA from Sphingobium japonicum UT26 catalyzes two steps of dehydrochlorination from gamma-hexachlorocyclohexane (gamma-HCH) to 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN) via gamma-pentachlorocyclohexene (gamma-PCCH). LinA was crystallized by the sitting-drop vapour-diffusion method using PEG 3350 as the precipitant. The crystals belonged to space group P4(1) or P4(3), with unit-cell parameters a = b = 68.9, c = 101.9 A, and diffracted X-rays to 2.25 A resolution. The crystal contained three molecules in the asymmetric unit.

Analysis of the role of LinA and LinB in biodegradation of ?-hexachlorocyclohexane

Commercial formulations of hexachlorocyclohexane (HCH) consist of a mixture of four isomers, alpha, beta, gamma and delta. All these four isomers are toxic and recalcitrant pollutants. Sphingobium (formerly Sphingomonas) sp. strain BHC-A is able to degrade all four HCH isomers. Eight lin genes responsible for the degradation of gamma-HCH in BHC-A were cloned and analysed for their role in the degradation of delta-HCH, and the initial conversion steps in delta-HCH catabolism by LinA and LinB in BHC-A were found. LinA dehydrochlorinated delta-HCH to produce 1,3,4,6-tetrachloro-1,4-cyclohexadiene (1,4-TCDN) via delta-pentachlorocyclohexene (delta-PCCH). Subsequently, both 1,4-TCDN and delta-PCCH are catalysed by LinB via two successive rounds of hydrolytic dechlorinations to form 2,5-dichloro-2,5-cyclohexadiene-1,4-diol (2,5-DDOL) and 2,3,5-trichloro-5-cyclohexene-1,4-diol (2,3,5-TCDL) respectively. LinB could also catalyse the hydrolytic dechlorination of delta-HCH to 2,3,5,6-tetrachloro-1,4-cyclohexanediol (TDOL) via 2,3,4,5,6-pentachlorocyclohexanol (PCHL).

Aerobic degradation of lindane (gamma-hexachlorocyclohexane) in bacteria and its biochemical and molecular basis

gamma-Hexachlorocyclohexane (gamma-HCH, also called gamma-BHC and lindane) is a halogenated organic insecticide that causes serious environmental problems. The aerobic degradation pathway of gamma-HCH was extensively revealed in bacterial strain Sphingobium japonicum (formerly Sphingomonas paucimobilis) UT26. gamma-HCH is transformed to 2,5-dichlorohydroquinone through sequential reactions catalyzed by LinA, LinB, and LinC, and then 2,5-dichlorohydroquinone is further metabolized by LinD, LinE, LinF, LinGH, and LinJ to succinyl-CoA and acetyl-CoA, which are metabolized in the citrate/tricarboxylic acid cycle. In addition to these catalytic enzymes, a putative ABC-type transporter system encoded by linKLMN is also essential for the gamma-HCH utilization in UT26. Preliminary examination of the complete genome sequence of UT26 clearly demonstrated that lin genes for the gamma-HCH utilization are dispersed on three large circular replicons with sizes of 3.5 Mb, 682 kb, and 191 kb. Nearly identical lin genes were also found in other HCH-degrading bacterial strains, and it has been suggested that the distribution of lin genes is mainly mediated by insertion sequence IS6100 and plasmids. Recently, it was revealed that two dehalogenases, LinA and LinB, have variants with small number of amino acid differences, and they showed dramatic functional differences for the degradation of HCH isomers, indicating these enzymes are still evolving at high speed.

Identification and characterization of genes encoding a putative ABC-type transporter essential for utilization of gamma-hexachlorocyclohexane in Sphingobium japonicum UT26

Sphingobium japonicum UT26 utilizes gamma-hexachlorocyclohexane (gamma-HCH) as its sole source of carbon and energy. In our previous studies, we cloned and characterized genes encoding enzymes for the conversion of gamma-HCH to beta-ketoadipate in UT26. In this study, we analyzed a mutant obtained by transposon mutagenesis and identified and characterized new genes encoding a putative ABC-type transporter essential for the utilization of gamma-HCH in strain UT26. This putative ABC transporter consists of four components, permease, ATPase, periplasmic protein, and lipoprotein, encoded by linK, linL, linM, and linN, respectively. Mutation and complementation analyses indicated that all the linKLMN genes are required, probably as a set, for gamma-HCH utilization in UT26. Furthermore, the mutant cells deficient in this putative ABC transporter showed (i) higher gamma-HCH degradation activity and greater accumulation of the toxic dead-end product 2,5-dichlorophenol (2,5-DCP), (ii) higher sensitivity to 2,5-DCP itself, and (iii) higher permeability of hydrophobic compounds than the wild-type cells. These results strongly suggested that LinKLMN are involved in gamma-HCH utilization by controlling membrane hydrophobicity. This study clearly demonstrated that a cellular factor besides catabolic enzymes and transcriptional regulators is essential for utilization of xenobiotic compounds in bacterial cells.

Biocatalytic dechlorination of lindane by nano-scale particles of Pd(0) deposited on Shewanella oneidensis

A new approach for the removal of the pesticide lindane (gamma-hexachlorocyclohexane or gamma-HCH) makes use of catalytic reduction of HCH to benzene over a metal catalyst, namely Pd(0). Since specific surface area plays an important role in reactivity of catalysts, this study investigated the use of bioPd(0), i.e. nano-scale Pd(0) particles precipitated on the biomass of Shewanella oneidensis, for the removal of lindane. It was demonstrated that bioPd(0) has catalytic activity towards dechlorination of gamma-HCH, with the addition of formate as electron donor, and that dechlorination with bioPd(0) was more efficient than with commercial powdered Pd(0). The biodegradable compound benzene was formed as reaction product and other HCH isomers could also be dechlorinated. Subsequently bioPd(0) was implemented in a membrane reactor technology for the treatment of gamma-HCH polluted water. In a fed-batch process configuration with formate as electron donor, a removal percentage of 98% of gamma-HCH saturated water (10 mg l(-1)) was achieved within 24h. The measured chloride mass balance approached the theoretical value. The results of this work showed that a complete, efficient and fast removal of lindane was achieved by biocatalysis with bioPd(0).

A gene linB2 responsible for the conversion of β-HCH and 2,3,4,5,6-pentachlorocyclohexanol in Sphingomonas sp. BHC-A

Commercial formulations of hexachlorocyclohexane (HCH) consist of a mixture of four isomers: alpha, beta, gamma, and delta. All four isomers are toxic and recalcitrant pollutants. beta-HCH is more problematic due to its longer persistence in the environment. Sphingomonas sp. BHC-A was able to degrade not only alpha-, gamma-, and delta-HCH but also beta-HCH. To clone a gene responsible for the degradation of beta-HCH, a Tn5 mutation was introduced into BHC-A, and one mutant BHC-A45 defective in beta-HCH degradation was selected. Sequencing analysis showed this mutant had a Tn5 insertion at the site of one haloalkane dehalogenase gene, designated linB2. linB2 was overexpressed in Escherichia coli and the 32-kDa product LinB2 showed the conversion activity of not only beta-HCH to beta-2,3,4,5,6-pentachlorocyclohexanol (beta-PCHL) but also beta-PCHL to beta-2,3,5,6-tetrachloro-1,4-cyclohexanediol.

Lindane removal induction byStreptomyces sp. M7

Gamma-hexachlorocyclohexane (gamma-HCH or lindane), one of the most commonly used pesticides, has been mainly used in agriculture; this pesticide is known to be highly toxic and persistent, causing serious water and soil contamination. The objective of the present work is to study the effect of low glucose concentration and the addition of lindane at different growing time on the pesticide detoxification ability of Streptomyces M7. After 96 h of incubation in synthetic medium containing glucose 0.6 g l(-1) with the addition of lindane 100 microg l(-1) at 20 h of incubation, a typical diauxic curve was obtained: glucose was the preferred substrate until 24 h, at 48 h, when the carbohydrate was depleted, the microorganism consumed the pesticide like carbon source. On the other hand, lindane removal induction was observed, which was greater when the pesticide was added to the medium at 20 h than 6 h of incubation. Between 72 and 96 h, a maximum of approximately 86% of the Cl(-) was released when lindane was added to the medium at 20 h, whereas approximately 70% and 67% Cl(-) was released in the medium when the pesticide was added at 0 and 6 h of incubation respectively. This is the first report of chloride release from inoculated medium supplemented with lindane, suggesting that the pesticide was degraded by Streptomyces sp. under aerobic conditions.

A glutathione S-transferase catalyzes the dehalogenation of inhibitory metabolites of polychlorinated biphenyls

BphK is a glutathione S-transferase of unclear physiological function that occurs in some bacterial biphenyl catabolic (bph) pathways. We demonstrated that BphK of Burkholderia xenovorans strain LB400 catalyzes the dehalogenation of 3-chloro 2-hydroxy-6-oxo-6-phenyl-2,4-dienoates (HOPDAs), compounds that are produced by the cometabolism of polychlorinated biphenyls (PCBs) by the bph pathway and that inhibit the pathway's hydrolase. A one-column protocol was developed to purify heterologously produced BphK. The purified enzyme had the greatest specificity for 3-Cl HOPDA (kcat/Km, approximately 10(4) M(-1) s(-1)), which it dechlorinated approximately 3 orders of magnitude more efficiently than 4-chlorobenzoate, a previously proposed substrate of BphK. The enzyme also catalyzed the dechlorination of 5-Cl HOPDA and 3,9,11-triCl HOPDA. By contrast, BphK did not detectably transform HOPDA, 4-Cl HOPDA, or chlorinated 2,3-dihydroxybiphenyls. The BphK-catalyzed dehalogenation proceeded via a ternary-complex mechanism and consumed 2 equivalents of glutathione (GSH) (Km for GSH in the presence of 3-Cl HOPDA, approximately 0.1 mM). A reaction mechanism consistent with the enzyme's specificity is proposed. The ability of BphK to dehalogenate inhibitory PCB metabolites supports the hypothesis that this enzyme was recruited to facilitate PCB degradation by the bph pathway.

Alg44, a unique protein required for alginate biosynthesis in Pseudomonas aeruginosa

Here the putative alginate biosynthesis gene alg44 of Pseudomonas aeruginosa was functionally assigned. Non-polar isogenic alg44 deletion mutants of P. aeruginosa were generated and did neither produce alginate nor released free uronic acids. No evidence for alginate enrichment in the periplasm was obtained. Alginate production was restored by introducing only the gene alg44. PhoA fusion protein analyses suggested that Alg44 is a soluble protein localized in the periplasm. Hexahistidine-tagged Alg44 was detected by immunoblotting. The corresponding 42.6 kDa protein was purified and identified by MALDI/TOF-MS analysis. Alg44 might be directly involved in alginate polymerization presumably by exerting a regulatory function.

Slow-release inoculation allows sustained biodegradation of gamma-hexachlorocyclohexane

This study investigated the feasibility of a slow-release inoculation approach as a bioaugmentation strategy for the degradation of lindane (gamma-hexachlorocyclohexane [gamma-HCH]). Slow-release inoculation of Sphingomonas sp. gamma 1-7 was established in both liquid and soil slurry microcosms using open-ended silicone tubes in which the bacteria are encapsulated in a protective nutrient-rich matrix. The capacity of the encapsulated cells to degrade lindane under aerobic conditions was evaluated in comparison with inoculation of free-living cells. Encapsulation of cells in tubes caused the removal of lindane by adsorption to the silicone tubes but also ensured prolonged biodegradation activity. Lindane degradation persisted 2.2 and 1.4 times longer for liquid and soil slurry microcosms, respectively, than that for inoculation with free cells. While inoculation of free-living cells led to a loss in lindane-degrading activity in limited time intervals, encapsulation in tubes allowed for a more stable actively degrading community. The loss in degrading activity was linked to the loss of the linA gene, encoding gamma-HCH dehydrochlorinase (LinA), which is involved in the initial steps of the lindane degradation pathway. This work shows that a slow-release inoculation approach using a catabolic strain encapsulated in open-ended tubes is a promising bioaugmentation tool for contaminated sites, as it can enhance pollutant removal and can prolong the degrading activity in comparison with traditional inoculation strategies.

Surveying biotransformations with a la carte genetic traps: translating dehydrochlorination of lindane (gamma-hexachlorocyclohexane) into lacZ-based phenotypes

The ability of the product of a desired reaction to activate a bacterial transcriptional regulator was exploited to develop genetic traps that render the catalytic activity born by a DNA clone into a selectable/scorable phenotype. We established this strategy with a system to expose the activity of dehydrochlorinases acting upon gamma-hexachlorocyclohexane (gamma-HCH or lindane). To this end, the effector-binding protein, XylR, was evolved by gene shuffling plus mutagenic polymerase chain reaction to be optimally responsive to the major product of gamma-HCH dehydrochlorination, 1,2,4-trichlorobenzene (TCB). We then derived Escherichia coli strains that constitutively expressed the modified XylR variant (named XylR5) and had lacZ under control of the Pu promoter, which is activated by XylR. A robotic beta-galactosidase assay indicated that when the resulting strain was transformed with a linA+ clone (expressing a gamma-HCH dehydrochlorinase from Sphingomonas paucimobilis UT26), it had levels of beta-galactosidase that were dependent on the gamma-HCH concentration. This à la carte host thus translated the conversion of gamma-HCH to TCB into upregulation of lacZ. An alternate host additionally expressing LacY grew efficiently on lactose only when LacZ was upregulated in a fashion dependent on TCB or other effectors of XylR5. These results demonstrated the power of deriving a host for the genetic scrutiny, rather than enzymatic screening, of clones expressing a given catabolic enzyme.

Degradation of beta-Hexachlorocyclohexane by Haloalkane Dehalogenase LinB from Sphingomonas paucimobilis UT26

Beta-Hexachlorocyclohexane (beta-HCH) is the most recalcitrant among the alpha-, beta-, gamma-, and delta-isomers of HCH and causes serious environmental pollution problems. We demonstrate here that the haloalkane dehalogenase LinB, reported earlier to mediate the second step in the degradation of gamma-HCH in Sphingomonas paucimobilis UT26, metabolizes beta-HCH to produce 2,3,4,5,6-pentachlorocyclohexanol.

Subcellular localization of pentachlorophenol 4-monooxygenase in Sphingobium chlorophenolicum ATCC 39723

We have studied the subcellular localization of pentachlorophenol 4-monooxygenase (PCP4MO) in Sphingobium chlorophenolicum ATCC 39723 during induction by pentachlorophenol (PCP). Using a monoclonal antibody CL6 specific to the native and recombinant PCP4MO, the enzyme was primarily found soluble as determined by immunoblot and ELISA analyses of cellular fractions. However, the enzyme was observed both in the soluble and membrane-bound forms during induction for 2-4 h, suggesting its translocation out from the cytoplasm. Electron microscopy confirmed that PCP4MO was predominantly present in the cytoplasm at 1 h, whereas at 4 h significant amount was detected also in the membrane and periplasm. After 6 h, the majority of PCP4MO was in the periplasm and only small amount was bound to the inner membrane or present in the cytoplasm. The results indicate that after biosynthesis PCP4MO in S. chlorophenolicum is exported via the inner membrane to the final location in the periplasm.

Cloning and characterization of lin genes responsible for the degradation of Hexachlorocyclohexane isomers by Sphingomonas paucimobilis strain B90

Hexachlorocyclohexane (HCH) has been used extensively against agricultural pests and in public health programs for the control of mosquitoes. Commercial formulations of HCH consist of a mixture of four isomers, alpha, beta, gamma, and delta. While all these isomers pose serious environmental problems, beta-HCH is more problematic due to its longer persistence in the environment. We have studied the degradation of HCH isomers by Sphingomonas paucimobilis strain B90 and characterized the lin genes encoding enzymes from strain B90 responsible for the degradation of HCH isomers. Two nonidentical copies of the linA gene encoding HCH dehydrochlorinase, which were designated linA1 and linA2, were found in S. paucimobilis B90. The linA1 and linA2 genes could be expressed in Escherichia coli, leading to dehydrochlorination of alpha-, gamma-, and delta-HCH but not of beta-HCH, suggesting that S. paucimobilis B90 contains another pathway for the initial steps of beta-HCH degradation. The cloning and characterization of the halidohydrolase (linB), dehydrogenase (linC and linX), and reductive dechlorinase (linD) genes from S. paucimobilis B90 revealed that they share approximately 96 to 99% identical nucleotides with the corresponding genes of S. paucimobilis UT26. No evidence was found for the presence of a linE-like gene, coding for a ring cleavage dioxygenase, in strain B90. The gene structures around the linA1 and linA2 genes of strain B90, compared to those in strain UT26, are suggestive of a recombination between linA1 and linA2, which formed linA of strain UT26.

Sec-dependent and Sec-independent translocation of haloacid dehalogenase Chd1 of Burkholderia cepacia MBA4 in Escherichia coli

2-Haloacid dehalogenases are hydrolytic enzymes that cleave the halogen-carbon bond(s) in haloalkanoic acids. We have previously isolated a cryptic haloacid dehalogenase gene from Burkholderia cepacia MBA4 and expressed it in Escherichia coli. This recombinant protein is unusual in having a long leader sequence, a property of periplasmic enzymes. In this paper, we report the functional role of this leader sequence. Western blot analyses showed that Chd1 is translocated to the periplasm. The results on the expression of Chd1 in the presence of sodium azide suggested the cleavage of the leader to be Sec-dependent. Chimeras of Chd1 and green fluorescent protein demonstrated that the leader sequence is fully functional in translocating the fusion protein to the periplasm. The expression of the chimeras in Sec mutants supported the Sec-dependent translocation. Surprisingly, recombinant Chd1 and a chimera with no leader sequence were also found in the periplasm.

Identification of protein fold and catalytic residues of gamma-hexachlorocyclohexane dehydrochlorinase LinA

gamma-Hexachlorocyclohexane dehydrochlorinase (LinA) is a unique dehydrochlorinase that has no homologous sequence at the amino acid-sequence level and for which the evolutionary origin is unknown. We here propose that LinA is a member of a novel structural superfamily of proteins containing scytalone dehydratase, 3-oxo-Delta(5)-steroid isomerase, nuclear transport factor 2, and the beta-subunit of naphthalene dioxygenase-all known structures with different functions. The catalytic and the active site residues of LinA are predicted on the basis of its homology model. Nine mutants that carry substitutions of the proposed catalytic residues were constructed by site-directed mutagenesis. In addition to these, eight mutants that have a potential to make contact with the substrate were prepared by site-directed mutagenesis. These mutants were expressed in Escherichia coli, and their activities in crude extract were evaluated. Most of the features of the LinA mutants could be explained on the basis of the present LinA model, indicating its validity. We conclude that LinA catalyzes the proton abstraction via the catalytic dyad H73-D25 by a similar mechanism as described for scytalone dehydratase. The results suggest that LinA and scytalone dehydratase evolved from a common ancestor. LinA may have evolved from an enzyme showing a dehydratase activity.

Colorimetric assay for Lindane dechlorination by bacteria

A colorimetric microtitre plate-based assay that detects haloalkane dehalogenase activity was modified to detect dechlorination of gamma-hexachlorocyclohexane (Lindane). Dechlorination is indicated by the colour change of phenol red from red to yellow, in a weakly buffered solution, as the solution becomes acidic due to HCl formed during dechlorination. Enzyme activity can be monitored by reading the absorbance of each well at 540 nm. Positive controls for the assay were the known Lindane-degrading microorganisms, Rhodanobacter lindaniclasticus and Sphingomonas paucimobilis UT26. Dechlorination in a scaled-up version of the assay was confirmed by GC/ECD detection of known metabolites of the test microorganisms from which the enzyme extracts were prepared. The assay was used to measure the rate of dechlorination in cell-free extracts of R. lindaniclasticus. It was also used to screen the cell-free extracts of 24 bacterial isolates, from a Lindane-contaminated soil, for Lindane dechlorination activity. Although no isolates tested positive, the assay represents a new inexpensive and rapid screening tool for the detection of Lindane-degrading microorganisms.

Microbial dehalogenation

Novel dehalogenases have been identified recently in various bacteria that utilise halogenated substrates. X-ray studies and sequence analysis have revealed insight into the molecular mechanisms of hydrolytic dehalogenases. Furthermore, genetic and biochemical studies have indicated that reductive dehalogenases are extra-cytoplasmic corrinoid-containing iron-sulphur proteins. Sequence analysis and mutagenesis studies indicate that several dehalogenases are homologous to enzymes that carry out transformations on non-halogenated substrates.

Secretion of nucleoside diphosphate kinase by mucoid Pseudomonas aeruginosa 8821: involvement of a carboxy-terminal motif in secretion

Nucleoside diphosphate kinase (Ndk) is a ubiquitous enzyme which functions in balancing the nucleotide pool of the cell. We have recently reported that in addition to being intracellular in both mucoid and nonmucoid Pseudomonas aeruginosa, Ndk is also secreted into the extracellular environment by mucoid P. aeruginosa cells. This secreted Ndk has biochemical activity similar to the intracellular Ndk and is 16 kDa in size. To demonstrate that Ndk is indeed secreted and to localize the secretion motif, we constructed an ndk knockout mutant, which lacks both intracellular and extracellular forms of Ndk. In this study, we report the construction of deletion derivatives made from the carboxy-terminal region of Ndk. These deletion derivatives were introduced into the ndk::Cm knockout mutant and were examined for the intracellular and extracellular presence of Ndk. It was observed that the carboxy-terminal 8-amino-acid region is required for the secretion of Ndk into the extracellular region. This region has the sequence DXXX, where X is a predominantly hydrophobic residue. Such sequences represent a conserved motif in proteins secreted by the type I secretory pathway in gram-negative microorganisms. To investigate the significance of this motif in the secretion of Ndk, we constructed a fusion protein of Ndk and the blue fluorescent protein (BFP) as well as a fusion protein of mutated Ndk (whose DTEV motif has been changed to AAAA) and the BFP. The presence of extracellular Ndk was detected only in the ndk::Cm knockout mutant harboring the wild-type BFP-Ndk protein fusion. We could not detect the presence of extracellular Ndk in the ndk::Cm knockout mutant containing the mutated BFP-Ndk protein fusion. In addition, we have also used immunofluorescence microscopy to localize the wild-type and mutated BFP-Ndk proteins in the cell. The significance of these observations is discussed.