phnE and glpT genes enhance utilization of organophosphates in Escherichia coli K-12

Toxicity assessment of chlorpyrifos on different organs of rat: exploitation of microbial-based enzymatic system for neutralization

This study was aiming to treat the chlorpyrifos (CPF), an organophosphate (OP) pesticide with microbial enzyme extract, and assess the toxicity effects of CPF before/after its treatment on the integrity of DNA (deoxyribonucleic acid) and the activities of enzymes AChE (acetylcholinestrase), GST (glutathione S-transferase), SOD (superoxide dismutase), CAT (catalase), and MDA (malondialdehyde) in different organs of rat. The untreated CPF in rat significantly increased the DNA damage and decreased the activities of all these enzymes. Among all the organs studied, the liver was the most affected organ. Further, CPF was treated with an OPH (organophosphate hydrolase) enzyme obtained from CPF degrading bacterial laboratory isolate Pseudomonas sp. (ChlD) to neutralize the toxicity of CPF. The crude intracellular enzyme extract degraded > 90% of added CPF and > 80% of its toxic intermediate 3,5,6-trichloropyridinol (TCP) which resulted in > 80% reduction of CPF toxicity in different organs of rat. Thus, this study not only illustrated the adverse effect of OPs on mammalian system but also suggested a highly efficient and eco-friendly way to remove the harmful pesticide from the environment and agricultural food products which may help to reduce the exposure of humans to such lethal toxicants.

Opine-based Agrobacterium competitiveness: dual expression control of the agrocinopine catabolism (acc) operon by agrocinopines and phosphate levels

Agrobacterium tumefaciens strain C58 can transform plant cells to produce and secrete the sugar-phosphate conjugate opines agrocinopines A and B. The bacterium then moves in response to the opines and utilizes them as exclusive sources of carbon, energy, and phosphate via the functions encoded by the acc operon. These privileged opine-involved activities contribute to the formation of agrobacterial niches in the environment. We found that the expression of the acc operon is induced by agrocinopines and also by limitation of phosphate. The main promoter is present in front of the first gene, accR, which codes for a repressor. This operon structure enables efficient repression when opine levels are low. The promoter contains two putative operators, one overlapping the -10 sequence and the other in the further upstream from it; two partly overlapped putative pho boxes between the two operators; and two consecutive transcription start sites. DNA fragments containing either of the operators bound purified repressor AccR in the absence of agrocinopines but not in the presence of the opines, demonstrating the on-off switch of the promoter. Induction of the acc operon can occur under low-phosphate conditions in the absence of agrocinopines and further increases when the opines also are present. Such opine-phosphate dual regulatory system of the operon may ensure maximum utilization of agrocinopines when available and thereby increase the chances of agrobacterial survival in the highly competitive environment with limited general food sources.

Microbial degradation of organophosphorus compounds

Synthetic organophosphorus compounds are used as pesticides, plasticizers, air fuel ingredients and chemical warfare agents. Organophosphorus compounds are the most widely used insecticides, accounting for an estimated 34% of world-wide insecticide sales. Contamination of soil from pesticides as a result of their bulk handling at the farmyard or following application in the field or accidental release may lead occasionally to contamination of surface and ground water. Several reports suggest that a wide range of water and terrestrial ecosystems may be contaminated with organophosphorus compounds. These compounds possess high mammalian toxicity and it is therefore essential to remove them from the environments. In addition, about 200,000 metric tons of nerve (chemical warfare) agents have to be destroyed world-wide under Chemical Weapons Convention (1993). Bioremediation can offer an efficient and cheap option for decontamination of polluted ecosystems and destruction of nerve agents. The first micro-organism that could degrade organophosphorus compounds was isolated in 1973 and identified as Flavobacterium sp. Since then several bacterial and a few fungal species have been isolated which can degrade a wide range of organophosphorus compounds in liquid cultures and soil systems. The biochemistry of organophosphorus compound degradation by most of the bacteria seems to be identical, in which a structurally similar enzyme called organophosphate hydrolase or phosphotriesterase catalyzes the first step of the degradation. organophosphate hydrolase encoding gene opd (organophosphate degrading) gene has been isolated from geographically different regions and taxonomically different species. This gene has been sequenced, cloned in different organisms, and altered for better activity and stability. Recently, genes with similar function but different sequences have also been isolated and characterized. Engineered microorganisms have been tested for their ability to degrade different organophosphorus pollutants, including nerve agents. In this article, we review and propose pathways for degradation of some organophosphorus compounds by microorganisms. Isolation, characterization, utilization and manipulation of the major detoxifying enzymes and the molecular basis of degradation are discussed. The major achievements and technological advancements towards bioremediation of organophosphorus compounds, limitations of available technologies and future challenge are also discussed.

Reversible phase variation in the phnE gene, which is required for phosphonate metabolism in Escherichia coli K-12

It is known that Escherichia coli K-12 is cryptic (Phn-) for utilization of methyl phosphonate (MePn) and that Phn+ variants can be selected for growth on MePn as the sole P source. Variants arise from deletion via a possible slip strand mechanism of one of three direct 8-bp repeat sequences in phnE, which restores function to a component of a putative ABC type transporter. Here we show that Phn+ variants are present at the surprisingly high frequency of >10(-2) in K-12 strains. Amplified-fragment length polymorphism analysis was used to monitor instability in phnE in various strains growing under different conditions. This revealed that, once selection for growth on MePn is removed, Phn+ revertants reappear and accumulate at high levels through reinsertion of the 8-bp repeat element sequence. It appears that, in K-12, phnE contains a high-frequency reversible gene switch, producing phase variation which either allows ("on" form) or blocks ("off" form) MePn utilization. The switch can also block usage of other metabolizable alkyl phosphonates, including the naturally occurring 2-aminoethylphosphonate. All K-12 strains, obtained from collections, appear in the "off" form even when bearing mutations in mutS, mutD, or dnaQ which are known to enhance slip strand events between repetitive sequences. The ability to inactivate the phnE gene appears to be unique to K-12 strains since the B strain is naturally Phn+ and lacks the inactivating 8-bp insertion in phnE, as do important pathogenic strains for which genome sequences are known and also strains isolated recently from environmental sources.

Detoxification of organophosphate nerve agents by immobilized Escherichia coli with surface-expressed organophosphorus hydrolase

An improved whole-cell technology for detoxifying organophosphate nerve agents was recently developed based on genetically engineered Escherichia coli with organophosphorus hydrolase anchored on the surface. This article reports the immobilization of these novel biocatalysts on nonwoven polypropylene fabric and their applications in detoxifying contaminated wastewaters. The best cell loading (256 mg cell dry weight/g of support or 50 mg cell dry weight/cm2 of support) and subsequent hydrolysis of organophosphate nerve agents were achieved by immobilizing nongrowing cells in a pH 8, 150 mM citrate-phosphate buffer supplemented with 1 mM Co2+ for 48 h via simple adsorption, followed by organophosphate hydrolysis in a pH 8, 50 mM citrate-phosphate buffer supplemented with 0.05 mM Co2+ and 20% methanol at 37 degrees C. In batch operations, the immobilized cells degraded 100% of 0.8 mM paraoxon, a model organophosphate nerve agent, in approximately 100 min, at a specific rate of 0.160 mM min-1 (g cell dry wt)-1. The immobilized cells retained almost 100% activity during the initial six repeated cycles and close to 90% activity even after 12 repeated cycles, extending over a period of 19 days without any nutrient supplementation. In addition to paraoxon, other commonly used organophosphates, such as diazinon, coumaphos, and methylparathion were hydrolyzed efficiently. The cell immobilization technology developed here paves the way for an efficient, simple, and cost-effective method for detoxification of organophosphate nerve agents.