Expression of the crystal protein gene under the control of the alpha-amylase promoter in Bacillus thuringiensis strains

Genetically modified entomopathogenic bacteria, recent developments, benefits and impacts: A review

Entomopathogenic bacteria (EPBs), insect pathogens that produce pest-specific toxins, are environmentally-friendly alternatives to chemical insecticides. However, the most important problem with EPBs application is their limited field stability. Moreover, environmental factors such as solar radiation, leaf temperature, and vapor pressure can affect the pathogenicity of these pathogens and their toxins. Scientists have conducted intensive research to overcome such problems. Genetic engineering has great potential for the development of new engineered entomopathogens with more resistance to adverse environmental factors. Genetically modified entomopathogenic bacteria (GM-EPBs) have many advantages over wild EPBs, such as higher pathogenicity, lower spraying requirements and longer-term persistence. Genetic manipulations have been mostly applied to members of the bacterial genera Bacillus, Lysinibacillus, Pseudomonas, Serratia, Photorhabdus and Xenorhabdus. Although many researchers have found that GM-EPBs can be used safely as plant protection bioproducts, limited attention has been paid to their potential ecological impacts. The main concerns about GM-EPBs and their products are their potential unintended effects on beneficial insects (predators, parasitoids, pollinators, etc.) and rhizospheric bacteria. This review address recent update on the significant role of GM-EPBs in biological control, examining them through different perspectives in an attempt to generate critical discussion and aid in the understanding of their potential ecological impacts.

Selection and characterization of Bacillus thuringiensis strains from northwestern Himalayas toxic against Helicoverpa armigera

In this study, we present the selection and the characterization of Bacillus thuringiensis (Bt) strains with respect to their cry/cyt gene content and toxicity evaluation toward one of the most important polyphagous lepidopteran pest, Helicoverpa armigera. Fifty-six Bt isolates were obtained from 10 different regions of northwestern Himalayas, recording a total B. thuringiensis index of 0.62. Scanning electron microscopy revealed presence of bipyramidal, spherical, flat and irregular crystal shapes; SDS-PAGE analysis of spore-crystal mixtures showed the prominence of 130, 70, and 100 kDa protein bands in majority of the isolates; PCR analysis with primers for eight cry and cyt gene families and 13 cry gene subfamilies resulted in isolates showing different combinations of insecticidal genes. Strains containing cry1 were the most abundant (57.1%) followed by cyt2 (46.42%), cry11 (37.5%), cry2 (28.57%), cry4 (21.42%), cyt1 (19.64%), cry3 (8.9%), and cry7, 8 (7.14%). A total of 30.35% of the strains did not amplify with any of the primers used in this study. Median lethal concentration 50 (LC50 ) estimates of spore-crystal mixtures of Bt-JK12, 17, 22, 48, and 72 against second instar larvae of H. armigera was observed to be 184.62, 275.39, 256.29, 259.93 μg ml-1 , respectively. B. thuringiensis presents great diversity with respect to the presence of crystal protein encoding genes and insecticidal activity. Four putative toxic isolates identified in this study have potential application in insect pest control. B. thuringiensis isolate JK12 exhibited higher toxicity against H. armigera than that of B. thuringiensis HD1, hence can be commercially exploited to control insect pest for sustainable crop production. The results of this study confirm the significance of continuous exploration of new Bt stains from different ecological regions of the world.

Sustained expression of keratinase gene under PxylA and PamyL promoters in the recombinant Bacillus megaterium MS941

The ker gene encoding pre-pro keratinase of Bacillus licheniformis MKU3 was cloned with xylose inducible promoter (PxylA) or alpha-amylase promoter (PamyL) or both in Escherichia coli-Bacillus shuttle vector, pWH1520 generating recombinant plasmids pWHK3, pWAK3 and pWXAK3 respectively. Compared with Bacillius megaterium MS941 (pWXAK3) expressing ker gene with PxylA-PamyL promoters, B. megaterium MS941 (pWAK3) with PamyL displayed higher keratinase yield (168.6 U/ml) and specific activity (14.59 U/mg) after 36 h of growth in LB medium, however the keratinase yield decreased in the culture grown in LB medium supplemented with starch or xylose or both. A maximum yield of 186.3 U/ml with specific activity of 17.25 U/mg was obtained from xylose induced keratinase expression in B. megaterium MS941 (pWHK3) grown for 24h. The recombinant plasmids were stably maintained with sustained expression of keratinase for about 60 generations in B. megaterium MS941 rather than in B. megaterium 1,4945.

Expression of the cry11A gene of Bacillus thuringiensis ssp. israelensis in Saccharomyces cerevisiae

The complete cry11A region gene of Bacillus thuringiensis ssp. israelensis was fused in frame to the 3' end of the GST gene under the control of the Saccharomyces cerevisiae HXK1 promoter. The fusion protein GST-cry11A was expressed in S. cerevisiae strain AMW13C+. The fusion gene GST-cry11A was expressed when yeast cells were grown on galactose and a nonfermentable medium containing ethanol as carbon and energy source. When the cells were grown in glucose, mannose, fructose, or glycerol as carbon sources, the GST-cry11A gene was repressed. Thus, a regulated expression in accordance with the regulatory activity of the HXK1 gene promoter has been detected. The GST-cry11A fusion protein was detected in the transformed yeasts as a soluble protein. The fusion protein was purified by affinity chromatography using glutathione-Sepharose beads. Cell-free extracts from transformed yeasts grown in ethanol-containing culture media showed insecticidal activity against third-instar Aedes aegypti larvae. This insecticidal activity was increased about 4-fold when the purified fusion protein was assayed.

Diversity of Bacteria and Archaea associated with a carbonate-rich metalliferous sediment sample from the Rainbow vent field on the Mid-Atlantic Ridge

Two sediment cores were collected in an inactive area of the deep-sea hydrothermal vent field Rainbow (36 degrees N on the Mid-Atlantic Ridge). Metals and carbonates were abundant throughout the cores; calcite (CaCO3) was found throughout the cores while dolomite [CaMg(CO3)2] and siderite (FeCO3) were only found in deeper layers. Using polymerase chain reaction (PCR)-amplified 16S rRNA gene sequence analysis, we examined the bacterial and archaeal diversity in a sediment layer that contained the three carbonates. The retrieved bacterial and archaeal sequences were new and less than 4% of the sequences exhibited 94% or more identity with that of cultured organisms. The analysis of the composition of the bacterial library revealed a high diversity of sequences. Half of the bacterial clones was affiliated to the gamma-Proteobacteria. Most of them had environmental sequences retrieved from deep-sea sediments as closest relatives, some of which being distantly related to free-living and symbiotic sulfur-oxidizers. Other sequences clustered in the alpha-, delta- and epsilon-Proteobacteria, the 'Bacteroidetes', the 'Planctomycetes', the 'Nitrospirae', the 'Actinobacteria', the 'Chlorobi ' and the 'Verrumicrobia'. Based on clonal abundance and sequence comparisons, phylotype groups putatively involved in the oxydation of sulfur compounds appeared to dominate in the studied sample. The majority of the archaeal sequences clustered in an euryarchaeotic lineage recently identified in the walls of black smokers suggesting a possible thermophilic way of life of these uncultured microorganisms. Oxygen isotopic composition of siderite and dolomite indicated that they were formed at 67 degrees C and 94 degrees C respectively. Together with chemical and microbiological data, this suggested that hydrothermal fluids may have circulated through this sediment.

Diversity of functional genes of methanogens, methanotrophs and sulfate reducers in deep-sea hydrothermal environments

To contribute to the identification of methanogens, methanotrophs and sulfate-reducing bacteria (SRB) in microbial communities from the 13 degrees N (East Pacific Rise) and Rainbow (Mid-Atlantic Ridge) hydrothermal vent fields, we investigated the diversity of mcrA, pmoA and dsrAB genes sequences. Clone libraries were obtained using DNA isolated from fragments of diffuse vents, sediment and in situ samplers. The clones were categorized by restriction fragment length polymorphism, and representatives of each group were sequenced. Sequences were related to that of hyperthermophilic (order Methanopyrales and family Methanocaldococcaceae), thermophilic and mesophilic (family Methanococcaceae) methanogens, thermophilic (proposed genus 'Methylothermus') and mesophilic type I methanotrophs, and hyperthermophilic (order Archaeoglobales), thermophilic (order Thermodesulfobacteriales) and mesophilic (family Desulfobulbaceae) SRB. Several of the obtained sequences were distantly related to the genes of cultivated organisms, providing evidence of the existence of novel lineages in the three functional groups. This study provides for the first time an insight into the diversity of several functional genes of deep-sea hydrothermal system microorganisms.

Bacillus thuringiensis and its pesticidal crystal proteins

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Engineering Bacillus thuringiensis bioinsecticides with an indigenous site-specific recombination system

The cry genes of Bacillus thuringiensis encode a diverse group of crystal-forming proteins that exhibit insecticidal activity, particularly against the larvae of lepidopteran, coleopteran, and dipteran insects. The efficacy of B. thuringiensis-based biopesticides may be improved through the genetic manipulation of these genes. A gene transfer system has been developed for the introduction and maintenance of cloned insecticidal cry genes on small plasmids in B. thuringiensis. This vector system combines a B. thuringiensis plasmid replicon and an indigenous site-specific recombination system that allows for the selective removal of ancillary or foreign DNA from the recombinant bacterium after introduction of the Cry-encoding plasmid. The site-specific recombination system is useful for engineering strains with unique combinations of cry genes, resulting in new active ingredients with improved insecticidal properties.

Synergistic activity of a Bacillus thuringiensis delta-endotoxin and a bacterial endochitinase against Spodoptera littoralis larvae

In an attempt to increase the insecticidal effect of the delta-endotoxin crystal protein CryIC on the relatively Cry-insensitive larvae of Spodoptera littoralis, a combination of CryIC and endochitinase was used. CryIC comprising the first 756 amino acids from Bacillus thuringiensis K26-21 and endochitinase ChiAII encoded by Serratia marcescens were separately produced in Escherichia coli carrying the genes in overexpression vectors. The endochitinase on its own, even at very low concentrations (0.1 microgram/ml), perforated the larval midgut peritrophic membrane. When applied together with low concentrations of CryIC, a synergistic toxic effect was obtained. In the absence of chitinase, about 20 micrograms of CryIC per ml was required to obtain maximal reduction in larval weight, while only 3.0 micrograms of CryIC per ml caused a similar toxic effect in the presence of endochitinase. Thus, a combination of the Cry protein and an endochitinase could result in effective insect control in transgenic systems in which the Cry protein is not expressed in a crystalline form.

Domain III substitution in Bacillus thuringiensis delta-endotoxin CryIA(b) results in superior toxicity for Spodoptera exigua and altered membrane protein recognition

To test our hypothesis that substitution of domain III of Bacillus thuringiensis delta-endotoxin (Cry) proteins might improve toxicity to pest insects, e.g., Spodoptera exigua, in vivo recombination was used to produce a number of cryIA(b)-cryIC hybrid genes. A rapid screening assay was subsequently exploited to select hybrid genes encoding soluble protoxins. Screening of 120 recombinants yielded two different hybrid genes encoding soluble proteins with domains I and II of CryIA(b) and domain III of CryIC. These proteins differed by only one amino acid residue. Both hybrid protoxins gave a protease-resistant toxin upon in vitro activation by trypsin. Bioassays showed that one of these CryIA(b)-CryIC hybrid proteins (H04) was highly toxic to S. exigua compared with the parental CryIA(b) protein and significantly more toxic than CryIC. In semiquantitative binding studies with biotin-labelled toxins and intact brush border membrane vesicles of S. exigua, this domain III substitution appeared not to affect binding-site specificity. However, binding to a 200-kDa protein by CryIA(b) in preparations of solubilized and blotted brush border membrane vesicle proteins was completely abolished by the domain III substitution. A reciprocal hybrid containing domains I and II of CryIC and domain III of CryIA(b) did bind to the 200-kDa protein, confirming that domain III of CryIA(b) was essential for this reaction. These results show that domain III of CryIC protein plays an important role in the level of toxicity to S. exigua, that substitution of domain III may be a powerful tool to increase the repertoire of available active toxins for pest insects, and that domain III is involved in binding to gut epithelium membrane proteins of S. exigua.