Dissolution and Degradation of Bacillus thuringiensis delta-Endotoxin by Gut Juice Protease of the Silkworm Bombyx mori

The crystal structure of insecticidal protein Txp40 from Xenorhabdus nematophila reveals a two-domain unique binary toxin with homology to the toxin-antitoxin (TA) system

Txp40 is a ubiquitous, conserved, and novel toxin from Xenorhabdus and Photorhabdus bacteria, toxic to a wide range of insect pests. However, the three-dimensional structure and toxicity mechanism for Txp40 or any of its sequence homologs are not yet known. Here, we are reporting the crystal structure of the insecticidal protein Txp40 from Xenorhabdus nematophila at 2.08 Å resolution. The Txp40 was structurally distinct from currently known insecticidal proteins. Txp40 consists of two structurally different domains, an N-terminal domain (NTD) and a C-terminal domain (CTD), primarily joined by a 33-residue long linker peptide. Txp40 displayed proteolytic propensity. Txp40 gets proteolyzed, removing the linker peptide, which is essential for proper crystal packing. NTD adopts a novel fold composed of nine amphipathic helices and has no shared sequence or structural homology to any known proteins. CTD has structural homology with RNases of type II toxin-antitoxin (TA) complex belonging to the RelE/ParE toxin domain superfamily. NTD and CTD were individually toxic to Galleria mellonella larvae. However, maximal toxicity was observed when both domains were present. Our results suggested that the Txp40 acts as a two-domain binary toxin, which is unique and different from any known binary toxins and insecticidal proteins. Txp40 is also unique because it belongs to the prokaryotic RelE/ParE toxin family with a toxic effect on eukaryotic organisms, in contrast to other members of the same family. Broad insect specificity and unique binary toxin complex formation make Txp40 a viable candidate to overcome the development of resistance in insect pests.

Processing Properties and Potency of Bacillus thuringiensis Cry Toxins in the Rice Leaffolder Cnaphalocrocis medinalis (Guenée)

Different Cry toxins derived from Bacillus thuringiensis (Bt) possess different insecticidal spectra, whereas insects show variations in their susceptibilities to different Cry toxins. Degradation of Cry toxins by insect midgut extracts was involved in the action of toxins. In this study, we explored the processing patterns of different Cry toxins in Cnaphalocrocis medinalis (Lepidoptera: Crambidae) midgut extracts and evaluated the impact of Cry toxins degradation on their potency against C. medinalis to better understand the function of midgut extracts in the action of different Cry toxins. The results indicated that Cry1Ac, Cry1Aa, and Cry1C toxins could be degraded by C. medinalis midgut extracts, and degradation of Cry toxins by midgut extracts differed among time or concentration effects. Bioassays demonstrated that the toxicity of Cry1Ac, Cry1Aa, and Cry1C toxins decreased after digestion by midgut extracts of C. medinalis. Our findings in this study suggested that midgut extracts play an important role in the action of Cry toxins against C. medinalis, and the degradation of Cry toxins by C. medinalis midgut extracts could reduce their toxicities to C. medinalis. They will provide insights into the action of Cry toxins and the application of Cry toxins in C. medinalis management in paddy fields.

Proteolysis activation of Cry1Ac and Cry2Ab protoxins by larval midgut juice proteases from Helicoverpa armigera

Proteolytic processing of Bacillus thuringiensis (Bt) Cry protoxins by insect midgut proteases is critical to their insecticidal activities against target insects. Although transgenic Bt cotton expressing Cry1Ac and Cry2Ab proteins have been widely used for control of the cotton bollworm (Helicoverpa armigera) in the field, the proteolytic cleavage sites in the two protoxins targeted by H. armigera midgut proteases are still not clear. In this study, the proteolysis of Cry1Ac and Cry2Ab protoxins by midgut juice prepared from midgut tissue of H. armigera larvae was investigated. Cleavage of Cry1Ac protoxin by midgut proteases formed a major protein fragment of ~65 kDa, and N-terminal sequencing revealed that cleavage occurred at Arg28 in the fore-end of helix α-1 in domain I of Cry1Ac. Cleavage of Cry2Ab protoxin by midgut juice proteases produced a major protein fragment of ~50 kDa, and the cleavage occurred at Arg139 between helices α-3 and α-4 in domain I of Cry2Ab. The amino acids Arg28 of Cry1Ac and Arg139 of Cry2Ab were predicted as putative trypsin cleavage sites. Bioassay data showed that the toxicities (LC₅₀s) of Cry1Ac and Cry2Ab protoxins were equivalent to those of their respective midgut juice-activated toxins in the susceptible SCD strain of H. armigera. Identification of the exact sites of N-terminal activation of Cry1Ac and Cry2Ab protoxins will provide a basis for a better understanding of the mode of action and resistance mechanisms based on aberrant activation of these protoxins in H. armigera.

Immuno-analytical method development for detection of transgenic Cry1Ac protein and its validation

BACKGROUND

Bacillus thuringiensis (Bt) synthesizes Cry1Ac protein, which is toxic to many lepidopteran pests, and the cry1ac gene has been expressed in several transgenic crop plants. The Cry1Ac protein has been isolated from Bt kurstaki HD73 and purified to homogeneity. Polyclonal antibodies were raised against purified Cry1Ac in rabbits and goat. Sandwich ELISA was developed for Cry1Ac using goat IgG as a coating antibody, and affinity-purified rabbit IgG as the primary antibody.

RESULTS

The sensitivity of the assay was in the range of 0.47-1000 ng. It was subsequently employed in validating biological samples. Fifteen different cotton-seed samples were screened: 12 were found to be Bt positive and 3 Bt negative. The CS7 seeds showed the highest Bt content of 8.51 ± 0.45 μg g^-1 , followed by CS8 (6.0 ± 0.02 μg g^-1 ), CS15 (5.9 ± 0.03 μg g^-1 ), CS9 (5.5 ± 0.05 μg g^-1 ), and CS10 (4.83 ± 0.013 μg g^-1 ). The CS5 seeds showed Bt content of 3.6 ± 0.21 μg g^-1 . The F2 generation, CS6 (Kaveri seeds) showed lower Bt content (2.9 ± 0.06 μg g^-1 ). The CL5 samples showed Cry1Ac content of 0.99 ± 0.009 μg g^-1 . The amount of Cry1Ac protein in leaves, stem, and roots of germinated Bt cotton plants (CS10 and CS4) were 1.76 ± 0.15 μg g^-1 , 1.9 ± 0.01 μg g^-1 , 2.0 ± 0.1 μg g^-1 , and 1.6 ± 0.15 μg g^-1 , 1.9 ± 0.01 μg g^-1 , and 2.0 ± 0.01 μg g^-1 dry tissue, respectively.

CONCLUSION

The method developed can be used for screening the expression levels of Cry1Ac in different transgenic Bt cultivars and also spurious Bt cotton seeds procured by farmers. © 2019 Society of Chemical Industry.

Impact of Q139R substitution of MEB4-Cry2Aa toxin on its stability, accessibility and toxicity against Ephestia kuehniella

The Bacillus thuringiensis subsp. kurstaki strain MEB4 was previously found to be highly toxic to Ephestia kuehniella. SDS-PAGE analysis of the recombinant strain DH5α (pBS-cry2Aa-MEB4) showed that Cry2Aa-MEB4 delta-endotoxins were forming inclusion bodies, and were 2.75 fold more toxic towards E. kuehniella than those of Cry2Aa-BNS3. Besides to the 65kDa active toxin, proteolysis activation of Cry2Aa-BNS3 protein with E. kuehniella midgut juice generated an extra proteolysis form of 49kDa, which was the result of another chymotrypsin cleavage located in Leu144. The amino acid sequences alignment of Cry2Aa-MEB4 and Cry2Aa-BNS3 showed that among the different 15 amino acids, the Q139R substitution was found to be interesting. In fact, due to its presence within the loop α3-α4, the chymotrypsin-like protease was unable to access to its site in Cry2Aa-MEB4, resulting to the production of only the 65kDa form. The accessible surface and the stability studies of the structure model of the Cry2Aa-BNS3-49 form showed a lower hydrophobicity surface due to the omission of 144 amino acids from the N-terminal comparing with the active Cry2Aa-MEB4 protein. All these features caused the diminishing of Cry2Aa-BNS3 toxicity towards E. kuehniella.

Highly toxic and broad-spectrum insecticidal local Bacillus strains engineered using protoplast fusion

Protoplast fusion was performed between a local Bacillus thuringiensis UV-resistant mutant 66/1a (Bt) and Bacillus sphaericus GHAI (Bs) to produce new Bacillus strains with a wider spectrum of action against different insects. Bt is characterized as sensitive to polymyxin and streptomycin and resistant to rifampicin and has shown 87% mortality against Spodoptera littoralis larvae at concentration of 1.5 × 10(7) cells/mL after 7 days of feeding; Bs is characterized as resistant to polymyxin and streptomycin and sensitive to rifampicin and has been shown to have 100% mortality against Culex pipiens after 1 day of feeding at the same concentration as that of Bt. Among a total of 64 Bt::Bs fusants produced on the selective medium containing polymyxin, streptomycin, and rifampicin, 17 fusants were selected because of their high mortality percentages against S. littoralis (Lepidoptera) and C. pipiens (Diptera). While Bt harboured 3 plasmids (600, 350, and 173 bp) and Bs had 2 plasmids (544 and 291 bp), all the selected fusants acquired plasmids from both parental strains. SDS-PAGE protein analysis of the 17 selected fusants and their parental strains confirmed that all fusant strains acquired and expressed many specific protein bands from the 2 parental strains, especially the larvicidal proteins to both lepidopteran and dipteran species with molecular masses of 65, 70, 80, 88, 100, and 135 kDa. Four protein bands with high molecular masses of 281, 263, 220, and 190 kDa, which existed in the Bt parental strain and did not exist in the Bs parental strain, and 2 other protein bands with high molecular masses of 185 and 180 kDa, which existed in the Bs parental strain and did not exist in the Bt parental strain, were expressed in most fusants. The results indicated the expression of some cry genes encoded for insecticidal crystal proteins from Bt and the binary toxin genes from Bs in all fusant strains. The recombinant fusants have more efficient and potential values for agricultural application compared with both the insecticidal Bt and the mosquitocidal Bs strains alone against S. littoralis and C. pipiens larvae, respectively.

Production of marker-free transgenic Jatropha curcas expressing hybrid Bacillus thuringiensis δ-endotoxin Cry1Ab/1Ac for resistance to larvae of tortrix moth (Archips micaceanus)

BACKGROUND

The potential biofuel plant Jatropha curcas L. is affected by larvae of Archips micaceanus (Walker), a moth of the family Tortricidae. The hybrid Bacillus thuringiensis (Bt) δ-endotoxin protein Cry1Ab/1Ac confers resistance to lepidopteran insects in transgenic rice.

RESULTS

Here, we report the production of a marker-free transgenic line of J. curcas (L10) expressing Cry1Ab/1Ac using Agrobacterium-mediated transformation and a chemically regulated, Cre/loxP-mediated DNA recombination system. L10 carries a single copy of marker-free T-DNA that contains the Cry1Ab/1Ac gene under the control of a maize phosphoenolpyruvate carboxylase gene promoter (P Pepc :Cry1Ab/1Ac:T Nos ). The P Pepc :Cry1Ab/1Ac:T Nos gene was highly expressed in leaves of L10 plants. Insecticidal bioassays using leaf explants of L10 resulted in 80-100% mortality of larvae of A. micaceanus at 4 days after infestation.

CONCLUSION

The results demonstrate that the hybrid Bt δ-endotoxin protein Cry1Ab/1Ac expressed in Jatropha curcas displays strong insecticidal activity to A. micaceanus. The marker-free transgenic J. curcas line L10 can be used for breeding of insect resistance to A. micaceanus.

Histopathology and the lethal effect of Cry proteins and strains of Bacillus thuringiensis Berliner in Spodoptera frugiperda J.E. Smith Caterpillars (Lepidoptera, Noctuidae)

Among the phytophagous insects which attack crops, the fall armyworm, Spodoptera frugiperda (J.E. Smith, 1797) (Lepidoptera, Noctuidae) is particularly harmful in the initial growth phase of rice plants. As a potential means of controlling this pest, and considering that the entomopathogen Bacillus thuringiensis Berliner demonstrates toxicity due to synthesis of the Cry protein, the present study was undertaken to evaluate this toxic effect of B. thuringiensis thuringiensis 407 (pH 408) and B. thuringiensis kurstaki HD-73 on S. frugiperda. The following method was used. Both bacterial strains were evaluated in vitro in 1st instar S. frugiperda caterpillars, by means of histopathological assays. The Cry1Ab and Cry1Ac proteins, codified by the respective strains of B. thuringiensis, were evaluated in vivo by bioassays of 1st instar S. frugiperda caterpillars in order to determine the Mean Lethal Concentration (LC50). The results of the histopathological analysis of the midget of S. frugiperda caterpillars demonstrate that treatment with the B. thuringiensis thuringiensis strain was more efficient, because the degradations of the microvilosities started 9 hours after treatment application (HAT), while in the B. thuringiensis kurstaki the same effect was noticed only after 12 HAT. Toxicity data of the Cry1Ab and Cry1Ac proteins presented for the target-species LC50 levels of 9.29 and 1.79 μg.cm-2 respectively. The strains and proteins synthesised by B. thuringiensis thuringiensis and B. thuringiensis kurstaki are effective in controlling S. frugiperda, and may be used to produce new biopesticides or the genes may be utilised in the genetic transformation of Oryza sativa L.

Toxic Trypsin Digest Fragment from the Bacillus thuringiensis Parasporal Protein

Enzymatic digestion in vitro of the Bacillus thuringiensis protoxin presumably releases and activates the toxin in a manner analogous to that which occurs when a B. thuringiensis sporulated fermentation preparation passes through the midgut of a lepidopteran larva. Therefore, a sporulated culture of B. thuringiensis subsp. kurstaki (serotype 3a3b) HD-263 was treated with trypsin to release an activated toxin soluble in bicarbonate buffer. A 63-kilodalton protein, toxic to cabbage looper larvae (Trichoplusia ni) and to lepidopteran cells in culture, was purified to homogeneity from this trypsin digest. The larvicide, a glycoprotein containing 5% carbohydrate (wt/wt), was purified from the soluble B. thuringiensis trypsin digest by using ammonium sulfate precipitation, anion-exchange chromatography, and hydrophobic-interaction chromatography. Its amino acid composition was high in nonpolar residues and unusually low in lysine and histidine. The isoelectric point was 6.5, and the amino acid on the N terminus was identified as isoleucine. The toxin was only slightly soluble in aqueous buffers unless the chaotropic agent potassium thiocyanate was added. Partial characterization of the toxin indicated that it corresponds well with reported sequences deduced from cloned genes.

A naturally occurring plant cysteine protease possesses remarkable toxicity against insect pests and synergizes Bacillus thuringiensis toxin

When caterpillars feed on maize (Zea maize L.) lines with native resistance to several Lepidopteran pests, a defensive cysteine protease, Mir1-CP, rapidly accumulates at the wound site. Mir1-CP has been shown to inhibit caterpillar growth in vivo by attacking and permeabilizing the insect's peritrophic matrix (PM), a structure that surrounds the food bolus, assists in digestion and protects the midgut from microbes and toxins. PM permeabilization weakens the caterpillar defenses by facilitating the movement of other insecticidal proteins in the diet to the midgut microvilli and thereby enhancing their toxicity. To directly determine the toxicity of Mir1-CP, the purified recombinant enzyme was directly tested against four economically significant Lepidopteran pests in bioassays. Mir1-CP LC₅₀ values were 1.8, 3.6, 0.6, and 8.0 ppm for corn earworm, tobacco budworm, fall armyworm and southwestern corn borer, respectively. These values were the same order of magnitude as those determined for the Bacillus thuringiensis toxin Bt-CryIIA. In addition to being directly toxic to the larvae, 60 ppb Mir1-CP synergized sublethal concentrations of Bt-CryIIA in all four species. Permeabilization of the PM by Mir1-CP probably provides ready access to Bt-binding sites on the midgut microvilli and increases its activity. Consequently, Mir1-CP could be used for controlling caterpillar pests in maize using non-transgenic approaches and potentially could be used in other crops either singly or in combination with Bt-toxins.

Proteolytic processing of native Cry1Ab toxin by midgut extracts and purified trypsins from the Mediterranean corn borer Sesamia nonagrioides

The proteolytic processing of native Cry1Ab toxin by midgut extracts from the Mediterranean corn borer, Sesamia nonagrioides, takes place in successive steps. Several cuts occur until a 74 kDa protein is obtained; this is further digested to give rise to an active form of 69 kDa, which can be again processed to fragments of 67, 66 and 43 kDa. We have shown that three different trypsins (TI, TIIA and TIII) purified from the S. nonagrioides midgut were able to digest Cry1Ab protoxin to obtain the active form of 69 kDa. Interestingly, TI and TIII further hydrolyzed the 69 kDa protein to a fragment of slightly lower molecular mass (67 kDa), while TIIA was able to continue digestion to give fragments of 46 and 43 kDa. These results contrast with those obtained using bovine trypsin, in which the main product of Cry1Ab digestion is a 69 kDa protein. The digestion of the toxin with a "non-trypsin" fraction from S. nonagrioides midgut lumen, mostly containing chymotrypsins and elastases and free of trypsin-like activity, resulted in a different processing pattern, yielding fragments of 79, 77, 71, 69 and 51 kDa. Our results indicate that trypsins and other proteases are involved in the first steps of protoxin processing, but trypsins play the most important role in obtaining the 74 and 69 kDa proteins. All the digestion products, including the proteins of 46 and 43 kDa obtained from the digestion of Cry1Ab by TIIA, were toxic to neonate larvae, indicating that none of the tested proteases contribute to toxin degradation in a significant manner.

Strategy for amplification and sequencing of insecticidal cry1A genes from Bacillus thuringiensis

A feasible and fully described strategy, with a detailed list of primers, for amplifying, cloning and sequencing known and potentially novel cry1A genes harboured by a Bacillus thuringiensis strain was successfully established. Based on the analysis of conserved regions of the cry1A genes, the 1AF and 1UR oligonucleotide primers were designed to amplify the whole open reading frame of these genes. The PCR products obtained revealed the successful amplification of cry1A genes from 13 B. thuringiensis strains. These bacteria were previously known to harbour at least one cry1A gene. An Argentinean B. thuringiensis isolate INTA Mo1-12 was randomly chosen for cloning and sequencing of cry1A genes by using a primer set developed in this study. Both nucleotide and amino acid sequences similarity analysis revealed that cry1Aa and cry1Ac from B. thuringiensis INTA Mo1-12 are new natural variants, showing several differences with the other known cry1A subclasses. These genes were named by the B. thuringiensis Pesticidal Crystal Protein Nomenclature Committee as cry1Aa15 and cry1Ac21 respectively.

Molecular and insecticidal characterization of a Cry1I protein toxic to insects of the families Noctuidae, Tortricidae, Plutellidae, and Chrysomelidae

The most notable characteristic of Bacillus thuringiensis is its ability to produce insecticidal proteins. More than 300 different proteins have been described with specific activity against insect species. We report the molecular and insecticidal characterization of a novel cry gene encoding a protein of the Cry1I group with toxic activity towards insects of the families Noctuidae, Tortricidae, Plutellidae, and Chrysomelidae. PCR analysis detected a DNA sequence with an open reading frame of 2.2 kb which encodes a protein with a molecular mass of 80.9 kDa. Trypsin digestion of this protein resulted in a fragment of ca. 60 kDa, typical of activated Cry1 proteins. The deduced sequence of the protein has homologies of 96.1% with Cry1Ia1, 92.8% with Cry1Ib1, and 89.6% with Cry1Ic1. According to the Cry protein classification criteria, this protein was named Cry1Ia7. The expression of the gene in Escherichia coli resulted in a protein that was water soluble and toxic to several insect species. The 50% lethal concentrations for larvae of Earias insulana, Lobesia botrana, Plutella xylostella, and Leptinotarsa decemlineata were 21.1, 8.6, 12.3, and 10.0 microg/ml, respectively. Binding assays with biotinylated toxins to E. insulana and L. botrana midgut membrane vesicles revealed that Cry1Ia7 does not share binding sites with Cry1Ab or Cry1Ac proteins, which are commonly present in B. thuringiensis-treated crops and commercial B. thuringiensis-based bioinsecticides. We discuss the potential of Cry1Ia7 as an active ingredient which can be used in combination with Cry1Ab or Cry1Ac in pest control and the management of resistance to B. thuringiensis toxins.

Characterization of a new Bacillus thuringiensis endotoxin, Cry47Aa, from strains that are toxic to the Australian sheep blowfly, Lucilia cuprina

Sixteen isolates of Bacillus thuringiensis, derived from various soil samples collected in Australia, are highly toxic to larvae of the sheep blowfly (Lucilia cuprina). The toxin gene from one of the strains (CAA890) was cloned by genome walking, and sequencing of the cloned fragments revealed a new cry gene, encoding a protein of 1134 amino acid residues, with a theoretical molecular mass of 139,209Da. Based on the amino acid sequence comparison with known Cry delta-endotoxins, the gene was designated cry47Aa. Homology modelling based on known crystal structures of the Cry toxins reveals the differences to be located in the loops of domain II in the putative toxin-receptor binding surfaces between Cry47Aa and the dipteran active Cry2Aa. We also showed that the cry47Aa gene is present in the other isolates that are highly toxic to the sheep blowfly.

A Bacillus thuringiensis Crystal Protein with Selective Cytocidal Action to Human Cells

Bacillus thuringiensis crystal proteins, well known to be toxic to certain insects but not pathogenic to mammals, are used as insecticidal proteins in agriculture and forest management. We here identified a crystal protein that is non-insecticidal and non-hemolytic but has strong cytocidal activity against various human cells with a markedly divergent target specificity, e.g. highly cytotoxic to HepG2 and Jurkat and less cytotoxic to the normal hepatocyte (HC) and HeLa. In slices of liver and colon cancer tissues, the toxin protein preferentially killed the cancer cells, leaving other cells unaffected. The cytocidal effect of the protein is non-apoptotic with swelling and fragmentation of the susceptible cells, although the apoptotic process does occur when the cell damage proceeded slowly. The amino acid sequence deduced from the nucleotide sequence of the cloned gene of the protein has little sequence homology with the insecticidal crystal proteins of B. thuringiensis. These observations raise the presence of a new group of the B. thuringiensis toxin and the possibility of new applications for the protein in the medical field.

Lectin-like binding ofBacillus thuringiensisvar,kurstakilepidopteran-specific toxin is an initial step in insecticidal action

The two delta-endotoxins comprising the Bacillus thuringiensis var. kurstaki HD1 insecticidal protein crystal were separated. The lepidopteran-specific protoxin was activated in vitro and its mechanism of action investigated. Toxicity towards Choristoneura fumiferana CF1 cells was specifically inhibited by preincubation of the toxin with N-acetylgalactosamine and N-acetylneuraminic acid. The lectins soybean agglutinin and wheat germ agglutinin, which bind N-acetylgalactosamine, also inhibited toxicity. Since N-acetylneuraminic acid is not known to occur in insects, these results suggest that the toxin may recognise a specific plasma membrane glycoconjugate receptor with a terminal N-acetylgalactosamine residue.

Structure of Cry2Aa suggests an unexpected receptor binding epitope

BACKGROUND

Genetically modified (GM) crops that express insecticidal protein toxins are an integral part of modern agriculture. Proteins produced by Bacillus thuringiensis (Bt) during sporulation mediate the pathogenicity of Bt toward a spectrum of insect larvae whose breadth depends upon the Bt strain. These transmembrane channel-forming toxins are stored in Bt as crystalline inclusions called Cry proteins. These proteins are the active agents used in the majority of biorational pesticides and insect-resistant transgenic crops. Though Bt toxins are promising as a crop protection alternative and are ecologically friendlier than synthetic organic pesticides, resistance to Bt toxins by insects is recognized as a potential limitation to their application.

RESULTS

We have determined the 2.2 A crystal structure of the Cry2Aa protoxin by multiple isomorphous replacement. This is the first crystal structure of a Cry toxin specific to Diptera (mosquitoes and flies) and the first structure of a Cry toxin with high activity against larvae from two insect orders, Lepidoptera (moths and butterflies) and Diptera. Cry2Aa also provides the first structure of the proregion of a Cry toxin that is cleaved to generate the membrane-active toxin in the larval gut.

CONCLUSIONS

The crystal structure of Cry2Aa reported here, together with chimeric-scanning and domain-swapping mutagenesis, defines the putative receptor binding epitope on the toxin and so may allow for alteration of specificity to combat resistance or to minimize collateral effects on nontarget species. The putative receptor binding epitope of Cry2Aa identified in this study differs from that inferred from previous structural studies of other Cry toxins.

Role of proteolysis in determining potency of Bacillus thuringiensis Cry1Ac delta-endotoxin

Bacillus thuringiensis protein delta-endotoxins are toxic to a variety of different insect species. Larvicidal potency depends on the completion of a number of steps in the mode of action of the toxin. Here, we investigated the role of proteolytic processing in determining the potency of the B. thuringiensis Cry1Ac delta-endotoxin towards Pieris brassicae (family: Pieridae) and Mamestra brassicae (family: Noctuidae). In bioassays, Cry1Ac was over 2,000 times more active against P. brassicae than against M. brassicae larvae. Using gut juice purified from both insects, we processed Cry1Ac to soluble forms that had the same N terminus and the same apparent molecular weight. However, extended proteolysis of Cry1Ac in vitro with proteases from both insects resulted in the formation of an insoluble aggregate. With proteases from P. brassicae, the Cry1Ac-susceptible insect, Cry1Ac was processed to an insoluble product with a molecular mass of approximately 56 kDa, whereas proteases from M. brassicae, the non-susceptible insect, generated products with molecular masses of approximately 58, approximately 40, and approximately 20 kDa. N-terminal sequencing of the insoluble products revealed that both insects cleaved Cry1Ac within domain I, but M. brassicae proteases also cleaved the toxin at Arg423 in domain II. A similar pattern of processing was observed in vivo. When Arg423 was replaced with Gln or Ser, the resulting mutant toxins resisted degradation by M. brassicae proteases. However, this mutation had little effect on toxicity to M. brassicae. Differential processing of membrane-bound Cry1Ac was also observed in qualitative binding experiments performed with brush border membrane vesicles from the two insects and in midguts isolated from toxin-treated insects.

Proteolytic processing of the Cyt1Ab1 toxin produced by Bacillus thuringiensis subsp. medellin

Bacillus thuringiensis produces d-endotoxins that require proteolytic processing to become active. The activation of the B. thuringiensis subsp. medellin 28 kDa (Cyt1Ab1) cytolytic toxin by trypsin, chymotrypsin and gut extract from Culex quinquefasciatus larvae was analyzed. The Cyt1Ab1 toxin of B. thuringiensis subsp. medellin was processed by all proteases tested to fragments between 23 and 25 kDa, while processing of the Cyt1Aa1 toxin produce fragments between 22.5 and 24.5 kDa. The Cyt1Ab1 toxin was preferentially processed at the alkaline pH of 12. The in vitro proteolytic processing of the Cyt1Ab1 toxin by C. quinquefasciatus larvae midgut extract showed a 25 kDa fragment; a similar result was observed when the activation was performed in the in vivo experiments. The solubilized Cyt1Ab1 toxin and the protease resistant cores generated by in vitro processing showed hemolytic activity but not mosquitocidal activity. Amino terminal sequence of the C. quinquefasciatus gut extract resistant fragment indicated that the cutting site was located between Lys31 and Asp32, with a sequence DDPNEKNNHNS; while for the trypsin-resistant fragment the cutting site was determined between Leu29 and Arg30, and for the chymotrypsin-resistant fragment between Arg30 and Lys31.

Activation pattern and toxicity of the Cry11Bb1 toxin of Bacillus thuringiensis subsp. medellin

Bacillus thuringiensis protoxins undergo proteolytic processing in the midgut of susceptible insects to become active. The ability to process the Cry11Bb1 protoxin by trypsin and Culex quinquefasciatus larval gut extracts was tested. The protease activity indicated by the appearance of proteolytic products increased with an increment in pH, with the highest activity being observed at pH 10.6. A time course study showed the proteolysis of the 94-kDa Cry11Bb protein ending with the production of fragments of relative molecular mass of 30 and 35 kDa within 5 min. In vitro, gut proteases extract cleaved the solubilized toxin between Ser59 and Ile60 and between Ala395 and Asn396, generating a 30-kDa N-terminal and a 35-kDa C-terminal fragment, respectively. Similarly, mosquito larvae processed in vivo the parasporal inclusions, generating the same fragments as those observed in vitro. The Cry11Bb1 protoxin activated with trypsin or gut proteases showed larvicidal activity against C. quinquefasciatus first instar larvae. The data suggest that gut proteases participate in the activation of CryllBbl protoxin, generating at least two different fragments on which the activity could reside.

Production of chymotrypsin-resistant Bacillus thuringiensis Cry2Aa1 delta-endotoxin by protein engineering

Cleavage of the Cry2Aa1 protoxin (molecular mass, 63 kDa) from Bacillus thuringiensis by midgut juice of gypsy moth (Lymantria dispar) larvae resulted in two major protein fragments: a 58-kDa fragment which was highly toxic to the insect and a 49-kDa fragment which was not toxic. In the midgut juice, the protoxin was processed into a 58-kDa toxin within 1 min, but after digestion for 1 h, the 58-kDa fragment was further cleaved within domain I, resulting in the protease-resistant 49-kDa fragment. Both the 58-kDa and nontoxic 49-kDa fragments were also found in vivo when (125)I-labeled toxin was fed to the insects. N-terminal sequencing revealed that the protease cleavage sites are at the C termini of Tyr49 and Leu144 for the active fragment and the smaller fragment, respectively. To prevent the production of the nontoxic fragment during midgut processing, five mutant proteins were constructed by replacing Leu144 of the toxin with Asp (L144D), Ala (L144A), Gly (L144G), His (L144H), or Val (L144V) by using a pair of complementary mutagenic oligonucleotides in PCR. All of the mutant proteins were highly resistant to the midgut proteases and chymotrypsin. Digestion of the mutant proteins by insect midgut extract and chymotrypsin produced only the active 58-kDa fragment, except that L144H was partially cleaved at residue 144.

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.

Role of DNA in the activation of the Cry1A insecticidal crystal protein from Bacillus thuringiensis

The Cry1A insecticidal crystal protein (protoxin) from six subspecies of Bacillus thuringiensis as well as the Cry1Aa, Cry1Ab, and Cry1Ac proteins cloned in Escherichia coli was found to contain 20-kilobase pair DNA. Only the N-terminal toxic moiety of the protoxin was found to interact with the DNA. Analysis of the crystal gave approximately 3 base pairs of DNA per molecule of protoxin, indicating that only a small region of the N-terminal toxic moiety interacts with the DNA. It is proposed that the DNA-protoxin complex is virus-like in structure with a central DNA core surrounded by protein interacting with the DNA with the peripheral ends of the C-terminal region extending outward. It is shown that this structure accounts for the unusual proteolysis observed in the generation of toxin in which it appears that peptides are removed by obligatory sequential cleavages starting from the C terminus of the protoxin. Activation of the protoxin by spruce budworm (Choristoneura fumiferana) gut juice is shown to proceed through intermediates consisting of protein-DNA complexes. Larval trypsin initially converts the 20-kilobase pair DNA-protoxin complex to a 20-kilobase pair DNA-toxin complex, which is subsequently converted to a 100-base pair DNA-toxin complex by a gut nuclease and ultimately to the DNA-free toxin.

Luminal proteinases from Plodia interpunctella and the hydrolysis of Bacillus thuringiensis CryIA(c) protoxin

The ability of proteinases in gut extracts of the Indianmeal moth, Plodia interpunctella, to hydrolyze Bacillus thuringiensis (Bt) protoxin, casein, and rho-nitroanilide substrates was investigated. A polyclonal antiserum to protoxin CryIA(c) was used in Western blots to demonstrate slower protoxin processing by gut enzymes from Bt subspecies entomocidus-resistant larvae than enzymes from susceptible or kurstaki-resistant strains. Enzymes from all three strains hydrolyzed N-alpha-benzoyl-L-arginine rho-nitroanilide, N-succinyl-ala-ala-pro-phenylalanine rho-nitroanilide, and N-succinyl-ala-ala-pro-leucine rho-nitroanilide. Zymograms and activity blots were used to estimate the apparent molecular masses, number of enzymes, and relative activities in each strain. Several serine proteinase inhibitors reduced gut enzyme activities, with two soybean trypsin inhibitors, two potato inhibitors, and chymostatin the most effective in preventing protoxin hydrolysis.

A protein complex from Choristoneura fumiferana gut-juice involved in the precipitation of delta-endotoxin from Bacillus thuringiensis subsp. sotto

A 75 kDa protein from spruce budworm (Choristoneura fumiferana) gut-juice has been isolated and shown to cause a specific precipitation of the delta-endotoxin from Bacillus thuringiensis subsp. sotto. This 75 kDa protein, separated by either column chromatography or SDS-PAGE, caused precipitation of the sotto toxin in both agarose diffusion gels and the PAGE gels. The precipitation event leads to limited proteolysis of the toxin and loss of larval toxicity. SDS-PAGE analysis of the precipitated toxin indicates that proteolysis of the toxin is not a prerequisite for precipitation. The protein responsible for precipitation, exhibits elastase-like activity and appears to be a complex which partially dissociates during boiling in SDS-PAGE sample buffer. Gut-juice from gypsy moth, forest tent caterpillar and white mark tussock moth also precipitated delta-endotoxin, but silkworm gut-juice gave a much weaker response. These results provide further evidence that, in the larval gut, differential processing of delta-endotoxin may play a role in the expression of activity towards various insect larvae.

Bacillus thuringiensis growth and toxicity. Basic and applied considerations

Despite the known importance of the composition of culture media and culture conditions on Bacillus thuringiensis growth and toxicity, very few reviews are concerned with this subject. This article reviews some aspects of the microbiology of Bacillus thuringiensis, and how toxicity is affected by the composition of growth media and bioreactor operation.

In vitro and in vivo proteolysis of the Bacillus thuringiensis subsp. israelensis CryIVD protein by Culex quinquefasciatus larval midgut proteases

Proteases with trypsin-, chymotrypsin- and thermolysin-like specificity were detected in Culex quinquefasciatus larval midguts. Their activities were monitored by N-terminal amino acid sequence analysis of the Bacillus thuringiensis subsp. israelensis CryIVD toxin proteolytic fragments. These proteases are located in the larval midgut and in different fractions obtained during the preparation of brush border membrane vesicles. The activity of the midgut proteases increased with an increase in pH. Both the chymotrypsin- and thermolysin-like activities are involved in the processing of solubilized CryIVD toxin, whereas an additional trypsin-like protease is necessary for the CryIVD parasporal inclusion processing. The solubilized CryIVD toxin was first cleaved between Thr347 and Phe348 and between Phe348 and Tyr349, generating a 40-kDa N-terminal fragment and a 32.5-kDa C-terminal fragment. The C-terminal domain was resistant to further processing, with only a small amount of a 31-kDa product appearing due to the action of a thermolysin-like protease. However, the N-terminal domain was very unstable, and was further degraded to about 30 kDa. Unlike the solubilized CryIVD toxin, the processing of the CryIVD parasporal inclusion was very slow at neutral pH. Three protease-resistant products were detected at pHs higher than 9.5 with an overnight incubation at 37 degrees C. The 30- and 28.5-kDa C-terminal peptides are proteolytic products of trypsin- and chymotrypsin-like proteases, respectively; while the 28-kDa N-terminal peptide has 27 amino acids deleted from the N-terminal end by a thermolysin-like protease.

Processing of delta-endotoxin from Bacillus thuringiensis subsp. kurstaki HD-1 and HD-73 by gut juices of various insect larvae

Midgut juices were prepared from Adoxophyes sp., smaller tea tortrix (STT); Bombyx mori, silkworm (SW); Spodoptera litura, common cutworm (CCW); Plutella xylostella, diamondback moth (DBM); and Musca domestica, housefly (HF) and immobilized onto Sepharose 4B. delta-Endotoxins (ICPs) from Bacillus thuringiensis subsp. kurstaki HD-1 and HD-73 were digested by these immobilized gut juice proteases. All gut juices tested derived relatively proteolytic resistant cores from ICP. The molecular sizes of these cores, about 55 kDa in SDS-PAGE, were resulted. In the case of CCW, however, digestion was very strong and only 1/20 concentration of core protein remained relative to other digests. The N-terminal amino acid sequencing of the core proteins showed that they were truncated at the very end of the N-terminus of protoxin, CryIA, at different sites. Although housefly larvae were completely insensitive to active toxin, the gut juice produced the core, suggesting that the housefly may lack the binding sites for the core-active toxin.

Isolation and partial characterization of binding proteins for immobilized delta endotoxin from solubilized brush border membrane vesicles of the silkworm, Bombyx mori, and the common cutworm, Spodoptera litura

1. Brush border membrane vesicles (BBMV) were prepared from the Bombyx mori, and Spodoptera litura, midguts. BBMV was solubilized. 2. Activated delta endotoxin from Bacillus thuringiensis were immobilized. 3. Solubilized BBMV proteins were applied to the toxin column and the proteins bound were analyzed by SDS-PAGE. 4. In the case of B. mori M(r) = 220,000, 150,000 and 130,000 and in the case of S. litura 160,000 bands were detected. 5. The bindings were inhibited by N-acetyl galactosamine and mannose. 6. The binding proteins applied onto a Con A column and eluted by 0.1 M methyl-alpha-glucose, suggesting that hybrid type sugar sidechain may involve in the interaction.

Characterization of the pH-mediated solubility of Bacillus thuringiensis var. san diego native delta-endotoxin crystals

Native crystals of Bacillus thuringiensis var. san diego, a coleopteran-specific delta-endotoxin, were metabolically labelled with [35S]methionine. Specific activity was 82,000 CPM/micrograms (2.44 Ci/mmol). Using a universal buffer formulated with the same ionic strength at every pH, we determined that native crystals dissolve above pH 10 and below pH 4. At the acidic pH, the rate of solubilization was substantially slower than at the alkaline pH. Recrystallization rates for the toxin were similar regardless of solubilization conditions. The banding patterns in denatured polyacrylamide gel electrophoresis were unaffected by solubilization conditions. Toxicity was higher for soluble toxin compared to crystal toxin, but virtually identical for the acidic and alkaline produced solutions. Acid solubilization is significant because of the acidic midgut of susceptible Coleoptera.

Bacillus thuringiensis crystal protein: effect of chemical modification of the cysteine and lysine residues

The 16 cysteine residues of reduced protoxin from Bacillus thuringiensis subsp. kurstaki HD-73 can be quantitatively reacted with: (a) iodoacetic acid, to give carboxymethyl protoxin; (b) iodoacetamide, giving carbaminomethyl protoxin and (c) N-(beta-iodoethyl)trifluoroacetamide to give aminoethyl protoxin. The carboxymethyl derivative was found to be significantly more soluble at neutral pH values where both the native protoxin and the carbaminomethyl derivative exhibit low solubilities. At the alkaline pH values (pH 9.5-10.5) normally used to solubilize the crystal protein, the native protein was slightly more soluble than either the carboxymethyl or the carbaminomethyl derivatives. The aminoethyl derivative had an extremely low solubility at all pH values. Succinic anhydride reacted with only 35% of the lysine residues in both the carboxymethyl and the carbaminomethyl protoxin derivatives. Nonetheless, these succinylated protoxins exhibited significantly increased solubilities at neutral pH values. All the derivatives were found to retain full insecticidal activity toward spruce budworm (Choristeneura fufimerana) larvae. It is concluded that all the cysteine residues and modified lysine residues are on the surface of the protein and that derivatization does not alter the conformation of the solubilized protoxin.

Bacillus sphaericus as a mosquito pathogen: properties of the organism and its toxins

In the course of sporulation, Bacillus sphaericus produces an inclusion body which is toxic to a variety of mosquito larvae. In this review we discuss the general biology of this species and concentrate on the genetics and physiology of toxin production and its processing in the midgut of the larval host. The larvicide of B. sphaericus is unique in that it consists of two proteins of 51 and 42 kDa, both of which are required for toxicity to mosquito larvae. There is a low level of sequence similarity between these two proteins, which differ in their sequences from all the other known insecticidal proteins of Bacillus thuringiensis. Within the midgut the 51- and 42-kDa proteins are processed to proteins of 43 and 39 kDa, respectively. The conversion of the 42-kDa protein to a 39-kDa protein results in a major increase in toxicity; the significance of the processing of the 51-kDa protein is not known. In contrast to the results with mosquito larvae, the 39-kDa protein is alone toxic for mosquito-derived tissue culture-grown cells, and this toxicity is not affected by the 51-kDa protein or its derivative, the 43-kDa protein. Comparisons of larvae from species which differ in their susceptibility to the B. sphaericus toxin indicate that the probable difference resides in the nature of the target sites of the epithelial midgut cells and not in uptake or processing of the toxin. A similar conclusion is derived from experiments involving tissue culture-grown cells from mosquito species which differ in their susceptibility to the B. sphaericus toxin.

Characterization of mosquitocidal activity of Bacillus thuringiensis subsp. fukuokaensis crystal proteins

The mosquitocidal crystals of Bacillus thuringiensis subsp. fukuokaensis were isolated and bioassayed against fourth-instar larvae of two mosquito species. The 50% lethal concentration values of the crystals to Aedes aegypti and Culex quinquefasciatus were 4.1 and 2.9 micrograms/ml, respectively. In addition, the solubilized crystals had hemolytic activity; 50 micrograms/ml was the lowest detectable level. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the crystals consisted of polypeptides of 90, 86, 82, 72, 50, 48, 37, and 27 kDa. When the solubilized inclusion was treated with C. quinquefasciatus midgut brush border membrane vesicles or Manduca sexta gut juice, only one major protein was detected. This protein retained mosquitocidal activity but had no detectable hemolytic activity. Immunological analysis of this subspecies and the subspecies israelensis, kyushuensis and darmstadiensis by using polyclonal antisera raised against the whole-crystal protein of B. thuringiensis subsp. fukuokaensis revealed that the proteins in subsp. fukuokaensis are distinct from proteins in the other subspecies because little cross-reaction was observed. Analysis of the plasmid pattern showed that the crystal protein genes are located on a plasmid of 130 MDa. Analysis of plasmid and chromosomal DNA from subsp. fukuokaensis showed little homology to the 72-kDa toxin gene (PG-14) of B. thuringiensis subsp. morrisoni. However, some of the proteins of B. thuringiensis subsp. fukuokaensis are homologous to other B. thuringiensis toxins because N-terminal amino acid analysis revealed that the 90-kDa protein is encoded by a cryIV gene type.

Activation and germination of Bacillus thuringiensis spores in Manduca sexta larval gut fluid

Incubation of Bacillus thuringiensis HD-1 spores in the larval gut fluid of Manduca sexta (tobacco hornworm) resulted in increased viable counts, conversion to phase-dark spores, and a loss of absorbance in spore suspensions, indicative of spore germination. Heat-activated and untreated spores incubated in water did not exhibit these changes. Only when spores were heat activated and incubated in germinants L-alanine and adenosine did changes in the spores approximate those observed in gut fluid. These data suggest that M. sexta larval gut fluid induces the activation and germination of B. thuringiensis spores.

Sporulation-associated activation of Bacillus sphaericus larvicide

Preparations of the larvicidal crystal from 46-h cultures of Bacillus sphaericus 2362 contain 125-, 110-, 63-, and 43-kilodalton (kDa) proteins (P. Baumann, B. M. Unterman, L. Baumann, A.H. Broadwell, S.J. Abbene, and R.D. Bowditch, J. Bacteriol. 163:738-747, 1985). The 63- and 43-kDa proteins, which have been purified, are not immunologically cross-reactive, and only the 43-kDa protein is toxic to mosquito larvae. Since antigenic determinants of the two smaller proteins have been detected in the higher-molecular-weight proteins (125 and 110 kDa), it has been suggested that the latter are precursors of the 43- and 63-kDa peptides. In the present study, purified 110-kDa protein was found to be toxic to the larvae of Culex pipiens (50% lethal concentration = 115 ng/ml). A luciferase-luciferin assay for intracellular ATP as well as an assay based on the exclusion of Trypan Blue by live cells indicated that the 110-kDa protein had no effect on tissue-culture-grown cells of C. quinquefasciatus, while cells exposed to the 43-kDa protein rapidly lost viability (50% lethal concentration = 54 microgram(s)/ml by the intracellular ATP assay). These findings suggested that the 110-kDa protein and, by extension, the 125-kDa protein are protoxins which are activated during sporulation by cleavage to a 43-kDa toxin. To further investigate the origins and relationships of the crystal proteins of B. sphaericus, we analyzed samples during the growth and sporulation of the culture. Synthesis of crystal proteins was initiated at the end of exponential growth and was completed after about 7 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural similarity between the lepidoptera- and diptera-specific insecticidal endotoxin genes of Bacillus thuringiensis subsp. "kurstaki" and "israelensis"

A gene from Bacillus thuringiensis subsp. "israelensis" was cloned from the large plasmids of this subspecies and was shown to code for a mosquitocidal polypeptide. The gene could be expressed in either Escherichia coli, Bacillus subtilis, or B. thuringiensis subsp. "israelensis" to produce the larvicidal activity. Similarly, a Lepidoptera-specific toxin gene from B. thuringiensis subsp. "kurstaki" was also cloned and expressed in E. coli and B. subtilis. Both cloned genes were sequenced and subjected to computer analysis. A long open translational reading frame coded for the B. thuringiensis subsp. "kurstaki" gene product. However, the B. thuringiensis subsp. "israelensis" clone was composed of two adjacent open reading frames oriented as if they were in a transcriptional operon. The products of the cloned genes retained their specificity for either Lepidoptera or Diptera. The control regions immediately preceding the toxin genes of both B. thuringiensis subspecies showed considerable DNA homology, most likely because both toxins are expressed only during sporulation. In addition, the deduced amino acid sequences from the two contiguous B. thuringiensis subsp. "israelensis" genes bore a striking resemblance to the deduced amino acid sequence from the single larger B. thuringiensis subsp. "kurstaki" gene, as if these two arrangements were evolutionarily related.

Mode of Action of Bipyramidal δ-Endotoxin of Bacillus thuringiensis subsp. kurstaki HD-1

The mode of action of the toxic fragment (P-59) derived from bipyramidal-shaped δ-endotoxin of Bacillus thuringiensis subsp. kurstaki HD-1 on the silkworm Bombyx mori was investigated. An enzyme-linked immunosorbent assay showed that there was no translocation of P-59 from the gut lumen to the hemocoel. When membrane vesicles prepared from silkworm midgut were incubated with P-59, normally smooth surface of vesicles became rough, and patch formation was observed on the surface. Vesicles treated with P-59 tended to agglutinate. The vesicle-denaturing activity of a 130,000-dalton subunit protein of bipyramidal toxin was enhanced by treatment with a gut juice protease of the silkworm. P-59 did not cause any uncoupling effect on mitochondria of the silkworm midgut. These results suggest that the attacking site of this toxin is not the mitochondrion but the cell membrane of the susceptible cell.

Isolation of a relatively nontoxic 65-kilodalton protein inclusion from the parasporal body of Bacillus thuringiensis subsp. israelensis

Ultrastructural studies of the mosquitocidal bacterium Bacillus thuringiensis subsp. israelensis revealed that the parasporal body contained three major inclusion types, designated types 1, 2, and 3, which could be differentiated on the basis of electron opacity and size and, to some extent, shape. The type-2 inclusion, which was of moderate electron density and often appeared as a bar-shaped polyhedral body, was isolated on NaBr gradients from purified parasporal bodies and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transmission electron microscopy, and bioassays against neonate larvae of Aedes aegypti. Purified inclusions averaged 150 to 200 nm by 500 to 700 nm in transverse sections and consisted almost exclusively of a 65-kilodalton (kDa) protein contaminated with minor quantities of 38- and 28-kDa proteins. Lethal concentration values at the 50% level for preparations of the purified parasporal body and the type-2 inclusion were, respectively, 0.66 and 43 ng/ml, indicating that the 65-kDa protein is only slightly toxic to mosquitoes in comparison to the intact parasporal body. Analysis of the type-2 polyhedral inclusion by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and bioassays during different stages of purification demonstrated a positive correlation between the toxicity of the preparation and the degree of contamination with the 28-kDa protein. These results indicate that the 65-kDa protein is not the primary larvicidal toxin, although it may act in conjunction with other parasporal body proteins to produce the high mosquitocidal toxicity characteristic of this bacterium.