Comparison of Disulfide Contents and Solubility at Alkaline pH of Insecticidal and Noninsecticidal Bacillus thuringiensis Protein Crystals

Performance insights into spray-dryer microencapsulated Bacillus thuringiensis cry pesticidal proteins with gum arabic and maltodextrin for effective pest control

Bacillus thuringiensis (Bt) produces crystals composed mainly of Cry pesticidal proteins with insecticidal activity against pests but are highly susceptible to degradation by abiotic factors. In this sense, encapsulation techniques are designed to improve their performance and lifetime. However, the effects of polymeric matrix encapsulation such as gum arabic and maltodextrin by spray-dryer in the mechanisms of action of Bt kurstaki and Bt aizawai are unknown. We analyzed crystal solubilization, protoxin activation, and receptor binding after microencapsulation and compared them with commercial non-encapsulated products. Microencapsulation did not alter protein crystal solubilization, providing 130 kDa (Cry1 protoxin) and 70 kDa (Cry2 protoxin). Activation with trypsin, chymotrypsin, and larval midgut juice was analyzed, showing that this step is highly efficient, and the protoxins were cleaved producing similar ~ 55 to 65 kDa activated proteins for both formulations. Binding assays with brush border membrane vesicles of Manduca sexta and Spodoptera frugiperda larvae provided a similar binding for both formulations. LC50 bioassays showed no significant differences between treatments but the microencapsulated treatment provided higher mortality against S. frugiperda when subjected to UV radiation. Microencapsulation did not affect the mechanism of action of Cry pesticidal proteins while enhancing protection against UV radiation. These data will contribute to the development of more efficient Bt biopesticide formulations. KEY POINTS: • Microencapsulation did not affect the mechanisms of action of Cry pesticidal proteins produced by Bt. • Microencapsulation provided protection against UV radiation for Bt-based biopesticides. • The study's findings can contribute to the development of more efficient Bt biopesticide formulations.

Importance of Cry Proteins in Biotechnology: Initially a Bioinsecticide, Now a Vaccine Adjuvant

A hallmark of Bacillus thuringiensis bacteria is the formation of one or more parasporal crystal (Cry) proteins during sporulation. The toxicity of these proteins is highly specific to insect larvae, exerting lethal effects in different insect species but not in humans or other mammals. The aim of this review is to summarize previous findings on Bacillus thuringiensis, including the characteristics of the bacterium, its subsequent contribution to biotechnology as a bioinsecticide due to the presence of Cry proteins, and its potential application as an adjuvant. In several studies, Cry proteins have been administered together with specific antigens to immunize experimental animal models. The results have shown that these proteins can enhance immunogenicity by generating an adequate immune response capable of protecting the model against an experimental infectious challenge, whereas protection is decreased when the specific antigen is administered without the Cry protein. Therefore, based on previous results and the structural homology between Cry proteins, these molecules have arisen as potential adjuvants in the development of vaccines for both animals and humans. Finally, a model of the interaction of Cry proteins with different components of the immune response is proposed.

How Does Bacillus thuringiensis Crystallize Such a Large Diversity of Toxins?

Bacillus thuringiensis (Bt) is a natural crystal-making bacterium. Bt diversified into many subspecies that have evolved to produce crystals of hundreds of pesticidal proteins with radically different structures. Their crystalline form ensures stability and controlled release of these major virulence factors. They are responsible for the toxicity and host specificity of Bt, explaining its worldwide use as a biological insecticide. Most research has been devoted to understanding the mechanisms of toxicity of these toxins while the features driving their crystallization have long remained elusive, essentially due to technical limitations. The evolution of methods in structural biology, pushing back the limits in size of amenable protein crystals now allows access to be gained to structural information hidden within natural crystals of such toxins. In this review, we present the main parameters that have been identified as key drivers of toxin crystallization in Bt, notably in the light of recent discoveries driven by structural biology studies. Then, we develop how the future evolution of structural biology will hopefully unveil new mechanisms of Bt toxin crystallization, opening the door to their hijacking with the aim of developing a versatile in vivo crystallization platform of high academic and industrial interest.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

The conserved cysteine residues in Bacillus thuringiensis Cry1Ac protoxin are not essential for the bipyramidal crystal formation

To evaluate the function of conserved cysteine residues in Cry1Ac protoxin, we constructed a series of Cry1Ac mutants in which single or multiple cysteine residues were replaced with serine. It was found that cysteine substitution had little effect on the protoxin expression and bipyramidal crystal formation. Bioassays using Plutella xylostella larvae showed that two mutants with fourteen cysteine residues in the C-terminal half and all sixteen residues replaced had similar toxicity as wildtype Cry1Ac protoxin. Our study suggests that the conserved cysteine resudues in the Cry1Ac protoxin are not essential for deposition into a bipyramidal crystal even though the C-terminal half was directly involved in crystal formation.

Solubility enhancement of Cry2Aa crystal through carboxy-terminal extension and synergism between the chimeric protein and Cry1Ac

It was reported that the highly conserved C-terminal region of Bacillus thuringiensis Cry1A protoxins was very important for parasporal crystal formation and solubility feature in alkaline environment. In order to improve the solubilization efficiency of Cry2Aa crystal, the coding sequences of Cry2Aa protein and the C-terminal half of Cry1Ac were fused seamlessly through Red/ET homologous recombination and expressed in an acrystalliferous B. thuringiensis strain under the control of the cry1Ac promoter and terminator. Microscopic observation revealed that the recombinant strain containing the chimeric gene cry2Aa-1Ac produced distinct parasporal inclusion with semispherical to approximately cuboidal shape during sporulation. SDS-PAGE analysis showed that this strain expressed stable 130-kDa Cry2Aa-1Ac chimeric protein, which was confirmed to be the correctly expressed product by LC-MS/MS. The chimeric protein inclusion could be effectively dissolved at pH 10.5 and activated by trypsin like the parental Cry1Ac crystal. While, the parental Cry2Aa crystal exhibited very low solubility under this condition. Bioassays against third-instar larvae of Helicoverpa armigera proved that the chimeric protein was more toxic than Cry2Aa. Additionally, synergistic effect was clearly detected between the chimeric protein and Cry1Ac against H. armigera, while there was only additive effect for the combination of wild Cry2Aa and Cry1Ac. These results indicated that the developed chimeric protein might serve as a potent insecticidal toxin used in the field against lepidopteran pests.

Effects and mechanisms of Bacillus thuringiensis crystal toxins for mosquito larvae

Bacillus thuringiensis is a Gram-positive aerobic bacterium that produces insecticidal crystalline inclusions during sporulation phases of the mother cell. The virulence factor, known as parasporal crystals, is composed of Cry and Cyt toxins. Most Cry toxins display a common 3-domain topology. Cry toxins exert intoxication through toxin activation, receptor binding and pore formation in a suitable larval gut environment. The mosquitocidal toxins of Bt subsp. israelensis (Bti) were found to be highly active against mosquito larvae and are widely used for vector control. Bt subsp. jegathesan is another strain which possesses high potency against broad range of mosquito larvae. The present review summarizes characterized receptors for Cry toxins in mosquito larvae, and will also discuss the diversity and effects of 3-D mosquitocidal Cry toxin and the ongoing research for Cry toxin mechanisms generated from investigations of lepidopteran and dipteran larvae.

Bacillus thuringiensis toxins: an overview of their biocidal activity

Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

Efficient production of Bacillus thuringiensis Cry1AMod toxins under regulation of cry3Aa promoter and single cysteine mutations in the protoxin region

Bacillus thuringiensis Cry1AbMod toxins are engineered versions of Cry1Ab that lack the amino-terminal end, including domain I helix α-1 and part of helix α-2. This deletion improves oligomerization of these toxins in solution in the absence of cadherin receptor and counters resistance to Cry1A toxins in different lepidopteran insects, suggesting that oligomerization plays a major role in their toxicity. However, Cry1AbMod toxins are toxic to Escherichia coli cells, since the cry1A promoter that drives its expression in B. thuringiensis has readthrough expression activity in E. coli, making difficult the construction of these CryMod toxins. In this work, we show that Cry1AbMod and Cry1AcMod toxins can be cloned efficiently under regulation of the cry3A promoter region to drive its expression in B. thuringiensis without expression in E. coli cells. However, p3A-Cry1Ab(c)Mod construction promotes the formation of Cry1AMod crystals in B. thuringiensis cells that were not soluble at pH 10.5 and showed no toxicity to Plutella xylostella larvae. Cysteine residues in the protoxin carboxyl-terminal end of Cry1A toxins have been shown to be involved in disulfide bond formation, which is important for crystallization. Six individual cysteine substitutions for serine residues were constructed in the carboxyl-terminal protoxin end of the p3A-Cry1AbMod construct and one in the carboxyl-terminal protoxin end of p3A-Cry1AcMod. Interestingly, p3A-Cry1AbMod C654S and C729S and p3A-Cry1AcMod C730S recover crystal solubility at pH 10.5 and toxicity to P. xylostella. These results show that combining the cry3A promoter expression system with single cysteine mutations is a useful system for efficient expression of Cry1AMod toxins in B. thuringiensis.

Isolation and characterization of entomopathogenic bacteria from soil samples from the western region of Cuba

The use of insect pathogens is a viable alternative for insect control because of their relative specificity and lower environmental impact. The search for wild strains against dipterans could have an impact on mosquito control programs. We have made an extensive screening of soil in western Cuba to find bacteria with larvicidal activity against mosquitoes. A total of 150 soil samples were collected and isolates were identifying using the API 50 CHB gallery. Phenotypic characteristics were analyzed by hierarchical ascending classification. Quantitative bioassays were conducted under laboratory conditions following the World Health Organization protocol in order to ascertain the toxicity and efficacy of isolates. The protein profiles of the crystal components were determined by SDS-PAGE. Eight hundred and eighty-one bacterial isolates were obtained, and 13 isolates with entomopathogenic activity were isolated from nine samples. Nine isolates displayed higher entomopathogenic activity against both Cx. quinquefasciatus and Ae. aegypti compared with the reference strain 266/2. All toxic isolates showed higher biological potency than the 266/2 strain. These isolates with high entomopathogenic activity displayed a protein pattern similar to the B. thuringiensis var. israelensis IPS-82 and 266/2 strains. These results are a valuable tool for the control of Diptera of medical importance.

Improvement of crystal solubility and increasing toxicity against Caenorhabditis elegans by asparagine substitution in block 3 of Bacillus thuringiensis crystal protein Cry5Ba

The crystal proteins from Bacillus thuringiensis are widely used for their specific toxicity against insects and nematodes. The highly conserved sequence blocks play an important role in Cry protein stability and flexibility, the basis of toxicity. The block 3 in Cry5Ba subfamily has a shorter sequence (only 12 residues) and more asparagine residues than that of others which harbor about 48 residues but only one asparagine. Based on the theoretical structure model of Cry5Ba, all three asparagines in block 3 are closely located in the interface of putative three domains, implying their probable importance in structure and function. In this study, all three asparagines in Cry5Ba2 block 3 were individually substituted with alanine by site-directed mutagenesis. The wild-type and mutant proteins were overexpressed and crystallized in acrystalliferous B. thuringiensis strain BMB171. However, the crystals formed in one of the mutants, designated N586A, abnormally disappeared and dissolved into the culture supernatant once the sporulation cells lysed, whereas the Cry5Ba crystal and the other mutant crystals were stable. The mutant N586A crystal, isolated from sporulation cells by the ultrasonic process, was found to be easily dissolved at wide range of pH value (5.0 to 10.0). Moreover, the toxicity assays showed that the mutant N586A exhibited nearly 9-fold-higher activity against nematodes and damaged the host's intestine more efficiently than the native Cry5Ba2. These data support the presumption that the amide residue Asn586 at the interface of domains might adversely affect the protein flexibility, solubility and resultant toxicity of Cry5Ba.

Bacillus thuringiensis HD-73 Spores Have Surface-Localized Cry1Ac Toxin: Physiological and Pathogenic Consequences

Spores from Cry(sup+) strains of Bacillus thuringiensis bound fluorescein isothiocyanate-labeled antibodies specific for the 65-kDa activated Cry 1Ac toxin, whereas spores from Bacillus cereus and Cry(sup-) strains of B. thuringiensis did not. The Cry(sup+) spores could be activated for germination by alkaline conditions (pH 10.3), whereas Cry(sup-) spores could not. Once the surrounding exosporia had been removed or permeabilized, Cry(sup+) spores were able to bind the toxin receptor(s) from insect gut brush border membrane vesicle preparations, and their germination rates were increased ca. threefold in the presence of brush border membrane vesicles. A model is presented whereby in the soil the Cry toxins on the spore surface are protected by the exosporium while in the gut they are exposed and available for binding to the insect receptors. This model explains why the disulfide-rich C terminus of the cry genes is so highly conserved even though it is removed during the processing of the protoxin to the activated toxin. It also highlights the trade-off resulting from having Cry toxins located on the spore surface, i.e., decreased spore resistance versus enhanced insect pathogenesis.

Targeted proteomics approach to species-level identification of Bacillus thuringiensis spores by AP-MALDI-MS

Anthrax infections progress at a rapid pace, making rapid detection methods of utmost importance. MALDI-MS proteomics methods focused on Bacillus anthracis detection have targeted chromosomally encoded proteins, which are highly conserved between closely related species, hindering species identification. Presented here is an AP-MALDI-MS method targeting plasmid-borne proteins from Bacillus spores for species-level identification. A bioinformatics analysis revealed that 60.3% and 75.4% of tryptic peptides from plasmid-borne proteins of B. anthracis and B. thuringiensis were species-specific, respectively. Reported here is a method in which plasmid-borne delta-endotoxins were extracted directly from B. thuringiensis spores in 100 mM KOH. The pH was then adjusted to 8 and a 5-min trypsin digestion was performed on the extracted proteins. The resulting tryptic peptides were analyzed by AP-MALDI-MS/MS, which produced a definitive identification the B. thuringiensis species-specific Cry1Ab protein with a MASCOT score of 278 and expect value of 7.5 x 10(-23). This method has demonstrated the detection and identification of B. thuringiensis spores at the species level following a 5-min trypsin digestion. The challenges in applying a similar approach to the detection of plasmid-borne protein toxins from B. anthracis are also discussed.

Biochemical and molecular characterization of delta-endotoxins in Bacillus thuringiensis

The delta-endotoxins (delta-ETX) of four native strains (RT7, RT19, RT25, and RT25), and one reference strain (4L1) of Bacillus thuringiensis were biochemically and molecularly characterized to determine their potential toxic activity against lepidopteran larvae. Crystals of delta-ETX were purified through a two-phase system to determine their morphology, molar mass, solubility, and resistance to proteinases. Toxic activity and cry gene content were also determined. Crystals from native strains exhibited polyhedral, irregular and cuboidal shapes, while those from 4L1 were bipyramidal. Seven proteins with estimated molar mass approximately 30-134 kDa were detected as the main components of the native delta-ETX. Only crystals from 4L1, RT24, and RT25 underwent complete solubilization at pH >12.0. Crystals from all strains produced trypsin-resistant peptides. None of the cry genes associated with toxicity in lepidopterans (cry1, cry2, cry9) was found in the native strains; however, 4L1 strain harbors cry1 and cry2 genes. Strains RT19 and RT25 caused significant mortality against Trichoplusia ni larvae with partial solubilization at pH 10, strain 4L1 caused 100 % mortality. Toxicity of native strains may come from a novel cry gene.

Novel isolate of Bacillus thuringiensis subsp. thuringiensis that produces a quasicuboidal crystal of Cry1Ab21 toxic to larvae of Trichoplusia ni

A new isolate (IS5056) of Bacillus thuringiensis subsp. thuringiensis that produces a novel variant of Cry1Ab, Cry1Ab21, was isolated from soil collected in northeastern Poland. Cry1Ab21 was composed of 1,155 amino acids and had a molecular mass of 130.5 kDa, and a single copy of the gene coding for this endotoxin was located on a approximately 75-kbp plasmid. When synthesized by the wild-type strain, Cry1Ab21 produced a unique, irregular, bipyramidal crystal whose long and short axes were both approximately 1 microm long, which gave it a cuboidal appearance in wet mount preparations. In diet incorporation bioassays, the 50% lethal concentrations of the crystal-spore complex were 16.9 and 29.7 microg ml(-1) for second- and fourth-instar larvae of the cabbage looper, Trichoplusia ni, respectively, but the isolate was essentially nontoxic to larvae of the beet armyworm, Spodoptera exigua. A bioassay of autoclaved spore-crystal preparations showed no evidence of beta-exotoxin activity, indicating that toxicity was due primarily to Cry1Ab21. Studies of the pathogenesis of isolate IS5056 in second-instar larvae of T. ni showed that after larval death the bacterium colonized and subsequently sporulated extensively throughout the cadaver, suggesting that other bacteria inhabiting the midgut lumen played little if any role in mortality. As T. ni is among the most destructive pests of vegetable crops in North America and has developed resistance to B. thuringiensis, this new isolate may have applied value.

Larval gut pH profile in pestiferous Chironomus crassicaudatus and Glyptotendipes paripes (Chironomidae: Diptera) in reference to the toxicity potential of Bacillus thuringiensis serovar israelensis

Gut pH and pH optimum of amylase were measured in 4th-stage larvae of 2 pestiferous chironomid species, Chironomus crassicaudatus and Glyptotendipes paripes. The gut pH in both species was close to neutral, varying from 6.7 to 7.4 and 6.9 to 7.6 pH units for C. crassicaudatus and G. paripes, respectively. In both species, slightly alkaline pH values were measured in the anterior part of the mesenteron, whereas slightly acidic pH values were recorded in gastric caecae and the posterior part of mesenteron. Amylase pH optimum in both chironomid species remained between pH values of 5.5 and 7, being closer to pH value 7 than 5.5. This profile was consistent with the measured values of the gut pH. The recorded larval gut pH profile in the 2 species of chironomids in the present study is much lower than gut pH reported for mosquitoes or Lepidoptera larvae. This could be the reason for the relatively much lower susceptibility of chironomid larvae to Bacillus thuringiensis serovar israelensis toxin proteins than some other nematoceran Diptera, specifically mosquitoes.

The mutation R423S in the Bacillus thuringiensis hybrid toxin CryAAC slightly increases toxicity for Mamestra brassicae L

Bacillus thuringiensis Cry1Ac toxin is 100 times less toxic than Cry1C to Mamestra brassicae. An R(423)S mutation abolishes Cry1Ac toxin proteolysis in M. brassicae gut juice but does not increase its toxicity to this insect. The CryAAC hybrid toxin (1Ac/1Ac/1Ca) is toxic to M. brassicae but is susceptible to gut protease digestion at the R(423) residue. Accordingly we have investigated the effect of the R(423)S mutation in CryAAC on its toxicity for M. brassicae and Pieris brassicae. Bioassays demonstrated that the R(423)S mutation slightly increased the toxicity of CryAAC for M. brassicae by having a significantly inhibitory effect on the growth of surviving larvae. The mutant hybrid was still highly toxic to P. brassicae. Features of CryAACR(423)S such as, (1) stability in M. brassicae gut juice and (2) crystal solubility were investigated. Computer simulations suggest that a possible major increase in flexibility in the CryAAC loop beta7/beta8 (G(391)-P(397)) caused by the R(423)S substitution could be a reason for the increase in M. brassicae toxicity.

Characterization of Cry34/Cry35 binary insecticidal proteins from diverse Bacillus thuringiensis strain collections

Bacillus thuringiensis crystal proteins of the Cry34 and Cry35 classes function as binary toxins showing activity on the western corn rootworm, Diabrotica virgifera virgifera LeConte. We surveyed 6,499 B. thuringiensis isolates by hybridization for sequences related to cry35A genes, identifying 78 strains. Proteins of the appropriate molecular mass (ca. 44 kDa) for Cry35 were observed in 42 of the strains. Full-length, or nearly full-length, sequences of 34 cry34 genes and 16 cry35 genes were also obtained from cloning, PCR analysis, and DNA sequencing. These included representatives of all known Cry34A, Cry34B, Cry35A, and Cry35B classes, as well as a novel Cry34A/Cry35A-like pair. Bioassay analysis indicated that cry35-hybridizing strains not producing a ca. 14-kDa protein, indicative of Cry34, were not active on corn rootworms, and that the previously identified Cry34A/Cry35A pairs were more active than the Cry34B/Cry35B pairs. The cry35-hybridizing B. thuringiensis strains were found in locales and materials typical for other B. thuringiensis strains. Comparison of the sequences with the geographic origins of the strains showed that identical, or nearly identical, sequences were found in strains from both Australasia and the Americas. Sequence similarity searches revealed that Cry34 proteins are similar to predicted proteins in Photorhabdus luminescens and Dictyostelium discoidium, and that Cry35Ab1 contains a segment similar to beta-trefoil domains that may be a binding motif. The binary Cry34/Cry35 B. thuringiensis crystal proteins thus appear closely related to each other, are environmentally ubiquitous, and share sequence similarities consistent with activity through membrane disruption in target organisms.

Trp132, Trp154, and Trp157 are essential for folding and activity of a Cyt toxin from Bacillus thuringiensis

Cyt2Aa2 is a cytolytic and mosquito larvicidal toxin produced by Bacillus thuringiensis subsp. darmstadiensis. The toxin contains 3 tryptophan residues at positions 132, 154, and 157. To study the role of tryptophan on protein structure and functions, each tryptophan residue was substituted by phenylalanine and other different amino acids. Expression test in Escherichia coli showed that all mutant proteins were highly produced as inclusion bodies similar to that of the wild type. The mutant W157F showed haemolytic and mosquito larvicidal activities comparable to the wild type but the mutant W157V and all other mutants at positions 132 and 154 have completely lost these activities. Solubilization and proteinase K activation tests indicated that aromatic residue is required at position 157 and tryptophan residues at positions 132 and 154 are critical residues playing important role to maintain structure and functions of the protein and cannot be changed to any other amino acid.

Expression and characterization of a recombinant Cry1Ac crystal protein with enhanced green fluorescent protein in acrystalliferous Bacillus thuringiensis

AIMS

To investigate fusion expression between Bacillus thuringiensis crystal protein and a foreign protein, the expression of a fusion protein comprised of Cry1Ac, and enhanced green fluorescent protein (EGFP) in B. thuringiensis Cry(-)B strain was examined.

METHODS AND RESULTS

The N-terminal fusion expression of EGFP in Cry1Ac was attempted under the control of the native cry1Ac promoter. The EGFP gene was cloned into pProMu and named pProMu-EGFP. The transformant, ProMu-EGFP/CB produced parasporal inclusions that were of bipyramidal-shaped crystals in size ranging from 200 to 300 nm. The fusion protein was approximately 150 kDa and identified by the immunoblot analysis using a Cry1Ac antibody and also a GFP antibody. The LC(50) of the ProMu-EGFP/CB was twofold higher when compared with that by the ProAc/CB. However, the crystal protein produced by the ProMu-EGFP/CB was effective on Plutella xylostella larvae.

CONCLUSIONS

The ProMu-EGFP/CB produced bipyramidal shaped and insecticidal crystals comprising fusion proteins.

SIGNIFICANCE AND IMPACT OF THE STUDY

Through the N-terminal fusion expression of EGFP and Cry1Ac, expression and crystallization between the B. thuringiensis crystal protein and a foreign protein were validated.

Molecular and insecticidal characterization of a Bacillus thuringiensis strain isolated during a natural epizootic

A new Bacillus thuringiensis strain belonging to the serovar aizawai was isolated from a dead larva of the lepidopteran Mythimna loreyi collected in a corn crop in Spain during a natural epizootic. This strain, which was named Leapi01, was compared with the kurstaki and aizawai strains isolated from Dipel(R) and Xentari(R), by electron microscopy, SDS-PAGE, plasmid pattern, PCR and insecticidal activity. This strain showed similar morphological and biochemical characteristics to the standard strains. The content in cry genes of Leapi01 was analysed with a set of general and specific primers recognizing most of the cry genes reported to date. DNA amplification was obtained with primers corresponding to six genes and, to clearly determine the identity of the genes, the amplified fragments were sequenced and corresponded to cry1Aa, cry1Ab, cry1Ca, cry1Da, cry2Ab and cry1Ia. However, the proteins encoded by two of these genes, Cry2 and Cry1I, were not detected in the SDS-PAGE of the purified parasporal bodies. The insecticidal activity of Leapi01 was determined by bioassays against two Lepidoptera species, Helicoverpa armigera and Spodoptera littoralis, that were found to be very susceptible to Leapi01 purified crystals. Since two of the cry genes identified in Leapi01 appear to be silent, other factors may be involved in the toxicity of the strain. As a result of this study, the potential of Leapi01 as biological control agent is discussed, with special emphasis on the high toxicity and relatively broad spectrum activity compared with two B. thuringiensis strains that are the active ingredients of commercial preparations commonly used as bioinsecticides.

Identification and characterization of the new bacillus thuringiensis serovars pirenaica (serotype H57) and iberica (serotype H59)

Two new Bacillus thuringiensis strains have been classified by the H antigen of the cells and differentiated by their morphological, biochemical and molecular characteristics. The flagellar agglutination showed that both strains bore specific H antigens which allowed their classification as the new serotypes H57 and H59. The serovar names proposed for the type strains characterized in this work are B. thuringiensis ser. pirenaica, for the H serotype 57, and B. thuringiensis ser. iberica, for the H serotype 59. Further characterization of these strains, by means of SDS-PAGE, Western inmunodetection, plasmid profile and cry-gene identification by polymerase chain reaction, confirmed the originality of the two novel serotypes. Toxicity tests carried out against several insect species, belonging to the orders Lepidoptera, Diptera and Coleoptera, showed no detectable insecticidal activity for either of the B. thuringiensis strains.

Optimization of Cry3A yields in Bacillus thuringiensis by use of sporulation-dependent promoters in combination with the STAB-SD mRNA sequence

The insecticidal activity of Bacillus thuringiensis strains toxic to coleopterous insects is due to Cry3 proteins assembled into small rectangular crystals. Toxin synthesis in these strains is dependent primarily upon a promoter that is active in the stationary phase and a STAB-SD sequence that stabilizes the cry3 transcript-ribosome complex. Here we show that significantly higher yields of Cry3A can be obtained by using dual sporulation-dependent cyt1Aa promoters to drive the expression of cry3Aa when the STAB-SD sequence is included in the construct. The Cry3A yield per unit of culture medium obtained with this expression system was 12.7-fold greater than that produced by DSM 2803, the wild-type strain of B. thuringiensis from which Cry3Aa was originally described, and 1.4-fold greater than that produced by NB176, a mutant of the same strain containing two or three copies of cry3Aa, which is the active ingredient of the commercial product Novodor, used for control of beetle pests. The toxicities of Cry3A produced with this construct or the wild-type strain were similar when assayed against larvae of the cottonwood leaf beetle, Chrysomela scripta. The volume of Cry3A crystals produced with cyt1Aa promoters and the STAB-SD sequence was 1.3-fold that of typical bipyramidal Cry1 crystals toxic to lepidopterous insects. The dual-promoter/STAB-SD system offers an additional method for potentially improving the efficacy of insecticides based on B. thuringiensis.

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.

Bacillus thuringiensis insecticidal proteins: molecular mode of action

Growing interest in biorational pesticides has placed the Bacillus thuringiensis insecticidal crystal proteins at the forefront of pesticides for plant genetic engineering. The development of improvement pesticides, both in enhanced activity and broader host range, depends on an understanding of its mechanism of action. This review presents a complete overview of the bacterium and the group of insecticidal proteins known as Cry proteins or delta-endotoxins. The molecular mode of action is described in detail, including the mapping of receptor binding sites by site-directed mutagenesis, the known receptors, and the ion-channel activity of the toxins.

Characterization of novel non-toxic Bacillus thuringiensis isolates from Korea

Four Bacillus Thuringiensis isolates from soil samples produced parasporal inclusions which were non-toxic to insects. The isolates were named B. thuringiensis NTB-1, NTB-2, NTB-3 and NTB-4. The parasporal inclusions were shown to be ovoid by phase contrast and scanning electron microscopy. The serotypes of the four isolates were determined by agglutination using 33 antisera; NTB-1 and NTB-4 seemed to be subsp. israelensis, and NTB-2 seemed to be subsp. pondicheriensis. NTB-3 did not react with the 33 antisera. However, comparison of parasporal protein and plasmid DNA patterns of the four isolates with those of 15 known non-toxic B. thuringiensis strains demonstrated that the four isolates are novel.

The Bacillus thuringiensis insecticidal toxin binds biotin-containing proteins

Brush border membrane vesicles from larvae of the tobacco hornworm, Manduca sexta, contain protein bands of 85 and 120 kDa which react directly with streptavidin conjugated to alkaline phosphatase. The binding could be prevented either by including 10 microM biotin in the reaction mixture or by prior incubation of the brush border membrane vesicles with an activated 60- to 65-kDa toxin from Bacillus thuringiensis HD-73. The ability of B. thuringiensis toxins to recognize biotin-containing proteins was confirmed by their binding to pyruvate carboxylase, a biotin-containing enzyme, as well as to biotinylated ovalbumin and biotinylated bovine serum albumin but not to their nonbiotinylated counterparts. Activated HD-73 toxin also inhibited the enzymatic activity of pyruvate carboxylase. The biotin binding site is likely contained in domain III of the toxin. Two highly conserved regions within domain III are similar in sequence to the biotin binding sites of avidin, streptavidin, and a biotin-specific monoclonal antibody. In particular, block 4 of the B. thuringiensis toxin contains the YAS biotin-specific motif. On the basis of its N-terminal amino acid sequence, the 120-kDa biotin-containing protein is totally distinct from the 120-kDa aminopeptidase N reported to be a receptor for Cry1Ac toxin.

Biochemical, immunological and toxicological characteristics of the crystal proteins of Bacillus thuringiensis subsp. medellin

Characterization of the insecticidal and hemolytic activity of solubilized crystal proteins of Bacillus thuringiensis (Bt) subsp. medellin (Btmed) was performed and compared to solubilized crystal proteins of isolates 1884 of B. thuringiensis subsp. israelensis (Bti) and isolate PG-14 of B. thuringiensis subsp. morrisoni (Btm). In general, at acid pH values solubilization of the Bt crystalline parasporal inclusions (CPI) was lower than at alkaline pH. The larvicidal activity demonstrated by the CPI of Btmed indicated that optimal solubilization of CPI takes place at a pH value of 11.3, in Bti at pH values from 5.03 to 11.3 and in Btm at pH values from 9.05 to 11.3 Hemolytic activity against sheep red blood cells was mainly found following extraction at pH 11.3 in all Bt strains tested. Polyacrylamide gel electrophoresis under denaturing conditions revealed that optimal solubilization of the CPI in all Bt strains takes place at the alkaline pH values from 9.05 to 11.3. An enriched preparation of Btmed crystals was obtained, solubilized and crystal proteins were separated on a size exclusion column (Sephacryl S-200). Three main protein peaks were observed on the chromatogram. The first peak had two main proteins that migrate between 90 to 100 kDa. These proteins are apparently not common to other Bt strains isolated to date. The second and third peaks obtained from the size exclusion column yielded polypeptides of 68 and 28-30 kDa, respectively. Each peak independently, showed toxicity against 1st instar Culex quinquefasciatus larvae. Interestingly, combinations of the fractions corresponding to the 68 and 30 kDa protein showed an increased toxicity. These results suggest that the 94 kDa protein is an important component of the Btmed toxins with the highest potency to kill mosquito larvae. When crystal proteins of Bti were probed with antisera raised independently against the three main protein fractions of Btmed, the only crystal protein that showed cross reaction was the 28 kDa protein. These data suggest that Btmed could be an alternative bacterium for mosquito control programs in case mosquito larval resistance emerges to Bti toxic proteins.