Nucleotide sequence of a coleopteran-active toxin gene from a new isolate of Bacillus thuringiensis subsp. tolworthi

Effective bacterial insecticidal proteins against coleopteran pests: A review

Coleoptera, the order of insects commonly referred to as beetles, are able to survive in various environments, and thus, comprise the largest order in the animal kingdom. Coleopterans mainly include coprophagous and phytophagous lineages, and many species of the latter lineage are serious pests. In addition to traditional chemical methods, biocontrol measures using various bacterial insecticidal proteins have also gradually been developed to control these insect pests. In this review, we summarized the possible coleopteran-pest-specific bacteria and insecticidal proteins that have been reported in the literature thus far and have provided a comprehensive overview and long-term guidance for the control of coleopteran pests in the future.

Engineering of Bacillus thuringiensis Cry Proteins to Enhance the Activity against Western Corn Rootworm

A novel Bacillus thuringiensis Cry protein, Cry8Hb, active against Diabrotica virgifera virgifera (Western corn rootworm, WCRW) was discovered. Unexpectedly, the anti-rootworm activity of the Cry8Hb toxin was enhanced significantly by fusing Escherichia coli maltose binding protein (MBP) to this Cry toxin. While the exact mechanism of the activity enhancement remains indefinite, it is probable that the enhancement is a result of increased solubility of the MBP-Cry8Hb fusion in the rootworm midgut. This hypothesis was examined using a synthetic Cry3 protein called IP3-1, which was not soluble at a neutral pH like Cry8Hb and marginally active to WCRW. When IP3-1 was fused to MBP, its anti-WCRW activity was enhanced 13-fold. To further test the hypothesis, DNA shuffling was performed on IP3-1 to increase the solubility without MBP. Screening of shuffled libraries found six new IP3 variants showing very high anti-WCRW activity without MBP. Sequence and 3D structure analysis of those highly active, shuffled IP3 variants revealed several charge-altering mutations such as Lys to Glu on the putative MBP-attaching side of the IP3 molecule. It is likely that those mutations make the protein acidic to substitute the functions of MBP including enhancing the solubility of IP3 at a neutral pH.

In Vivo Crystallization of Three-Domain Cry Toxins

Bacillus thuringiensis (Bt) is the most successful, environmentally-friendly, and intensively studied microbial insecticide. The major characteristic of Bt is the production of proteinaceous crystals containing toxins with specific activity against many pests including dipteran, lepidopteran, and coleopteran insects, as well as nematodes, protozoa, flukes, and mites. These crystals allow large quantities of the protein toxins to remain stable in the environment until ingested by a susceptible host. It has been previously established that 135 kDa Cry proteins have a crystallization domain at their C-terminal end. In the absence of this domain, Cry proteins often need helper proteins or other factors for crystallization. In this review, we classify the Cry proteins based on their requirements for crystallization.

The red flour beetle as a model for bacterial oral infections

Experimental infection systems are important for studying antagonistic interactions and coevolution between hosts and their pathogens. The red flour beetle Tribolium castaneum and the spore-forming bacterial insect pathogen Bacillus thuringiensis (Bt) are widely used and tractable model organisms. However, they have not been employed yet as an efficient experimental system to study host-pathogen interactions. We used a high throughput oral infection protocol to infect T. castaneum insects with coleopteran specific B. thuringiensis bv. tenebrionis (Btt) bacteria. We found that larval mortality depends on the dietary spore concentration and on the duration of exposure to the spores. Furthermore, differential susceptibility of larvae from different T. castaneum populations indicates that the host genetic background influences infection success. The recovery of high numbers of infectious spores from the cadavers indicates successful replication of bacteria in the host and suggests that Btt could establish infectious cycles in T. castaneum in nature. We were able to transfer plasmids from Btt to a non-pathogenic but genetically well-characterised Bt strain, which was thereafter able to successfully infect T. castaneum, suggesting that factors residing on the plasmids are important for the virulence of Btt. The availability of a genetically accessible strain will provide an ideal model for more in-depth analyses of pathogenicity factors during oral infections. Combined with the availability of the full genome sequence of T. castaneum, this system will enable analyses of host responses during infection, as well as addressing basic questions concerning host-parasite coevolution.

A novel cadherin-like gene from western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), larval midgut tissue

A cadherin-like gene associated with larval midgut tissues was cloned from western corn rootworm (Diabrotica virgifera virgifera: Coleoptera), an economically important agricultural pest in North America and Europe and the primary target pest species for corn hybrids expressing Cry3 toxins from Bacillus thuringiensis (Bt). The full-length cDNA (5371 bp in length) encodes an open reading frame for a 1688 amino acid polypeptide. The putative protein has similar architecture to cadherin-like proteins isolated from lepidopteran midguts that have been shown to bind to Cry1 Bt toxins and have been implicated in Bt resistance. The D. v. virgifera cadherin-like gene is expressed primarily in the larval midgut and regulated during development, with high levels of expression observed in all instars and adults but not pupae. The corresponding genomic sequence spans more than 90 kb and is interspersed with 30 large introns. The genomic organization of the cadherin-like gene for this coleopteran species bears strong resemblance to lepidopteran cadherins suggesting a common molecular basis for susceptibility to Cry3 toxins in Coleoptera.

Typical Three-Domain Cry Proteins of Bacillus thuringiensis Strain A1462 Exhibit Cytocidal Activity on Limited Human Cancer Cells

Bacillus thuringiensis strain A1462 produced two parasporal inclusion proteins with a molecular mass of 88 kDa that were converted to 64-kDa toxins when activated by proteinase K digestion. Both toxins exhibited strong cytocidal activity against two human cancer cell lines, HL60 (myeloid leukemia cells) and HepG2 (liver cancer cells), while low or no toxicities were observed against 11 human and three mammalian cell lines, including four non-cancer cell lines. The cytotoxicity of both toxins on susceptible cells was characterized by rapid cell swelling. Gene cloning experiments provided two novel genes encoding 88-kDa Cry proteins, Cry41Aa and Cry41Ab. The amino acid sequences of the two proteins contain five block regions commonly conserved in B. thuringiensis insecticidal Cry proteins. This is the first report of the occurrence of typical three-domain Cry proteins with cytocidal activity preferential for cancer cells.

Role of toxin activation on binding and pore formation activity of the Bacillus thuringiensis Cry3 toxins in membranes of Leptinotarsa decemlineata (Say)

Binding and pore formation constitute key steps in the mode of action of Bacillus thuringiensis delta-endotoxins. In this work, we present a comparative analysis of toxin-binding capacities of proteolytically processed Cry3A, Cry3B and Cry3C toxins to brush border membranes (BBMV) of the Colorado potato beetle Leptinotarsa decemlineata (CPB), a major potato coleopteran-insect pest. Competition experiments showed that the three Cry3 proteolytically activated toxins share a common binding site. Also heterologous competition experiments showed that Cry3Aa and Cry3Ca toxins have an extra binding site that is not shared with Cry3Ba toxin. The pore formation activity of the three different Cry3 toxins is analysed. High pore-formation activities were observed in Cry3 toxins obtained by proteolytical activation with CPB BBMV in contrast to toxins activated with either trypsin or chymotrypsin proteases. The pore-formation activity correlated with the formation of soluble oligomeric structures. Our data support that, similarly to the Cry1A toxins, the Cry3 oligomer is formed after receptor binding and before membrane insertion, forming a pre-pore structure that is insertion-competent.

Bacillus thuringiensis cry3Ca1 protein is toxic to the Colorado potato beetle, Leptinotarsa decemlineata (Say)

We expressed the wild-type cry3Aa3 and cry3Ca1 Bacillus thuringiensis genes, which code for insecticidal proteins, in an Escherichia coli expression system. Highly purified preparations of the soluble delta-endotoxins were used to perform comparative bioassays with third-instar larvae of the Colorado potato beetle (CPB). Acute mortality data showed that Cry3Ca1 (LD(50) = 320.1 ng) was 2-fold more toxic than Cry3Aa3 (LD(50) = 672.9 ng). We also compared the chronic effects of sublethal doses of these toxins by measuring the consumption of untreated foliage and monitoring survival and development for 6 days after intoxication. No significant additional mortality was recorded, but we found that surviving larvae fed Cry3Ca1 consumed foliage at a slower rate than the larvae fed Cry3Aa3, suggesting more damage to their digestive epithelium. This study, the first assessment of the toxicity of cry3Ca1 in third-instar CPB, suggests cry3Ca1 will prove useful for the control of this important insect pest.

Screening and characterization of Bacillus thuringiensis isolates from Brazil for the presence of coleoptera-specific cry genes

Forty-three Bacillus thuringiensis isolates from Brazil and 3 from Argentina were screened, using the polymerase chain reaction (PCR), for various coleoptera-specific cry genes. Seven isolates produced specific and/or nonspecific DNA fragments in a PCR reaction with primers specific for two coleopteran cry genes and 4 of these produced DNA fragments with primers specific for 7 known coleopteran cry genes. These isolates showed, by electron microscopy, the presence of spherical crystals. They also showed proteins of around 70 kDa which were immunologically similar to the Cry3Aa protein from B. thuringiensis subsp. tenebrionis. The 3 isolates from Argentina were toxic to T. molitor, and although no isolate from Brazil showed toxicity, they might show toxicity to another insect species.

Revision of the nomenclature for the Bacillus thuringiensis pesticidal crystal proteins

The crystal proteins of Bacillus thuringiensis have been extensively studied because of their pesticidal properties and their high natural levels of production. The increasingly rapid characterization of new crystal protein genes, triggered by an effort to discover proteins with new pesticidal properties, has resulted in a variety of sequences and activities that no longer fit the original nomenclature system proposed in 1989. Bacillus thuringiensis pesticidal crystal protein (Cry and Cyt) nomenclature was initially based on insecticidal activity for the primary ranking criterion. Many exceptions to this systematic arrangement have become apparent, however, making the nomenclature system inconsistent. Additionally, the original nomenclature, with four activity-based primary ranks for 13 genes, did not anticipate the current 73 holotype sequences that form many more than the original four subgroups. A new nomenclature, based on hierarchical clustering using amino acid sequence identity, is proposed. Roman numerals have been exchanged for Arabic numerals in the primary rank (e.g., Cry1Aa) to better accommodate the large number of expected new sequences. In this proposal, 133 crystal proteins comprising 24 primary ranks are systematically arranged.

Identification of novel cry-type genes from Bacillus thuringiensis strains on the basis of restriction fragment length polymorphism of the PCR-amplified DNA

Two pairs of universal oligonucleotide primers were designed to probe the most conserved regions of all known cryI-type gene sequences so that the amplified PCR fragments of the DNA template from Bacillus thuringiensis strains may contain all possible cryI-type gene sequences. The restriction fragment length polymorphism (RFLP) patterns of the PCR-amplified fragments revealed that 14 distinct cry-type genes have been identified from 20 B. thuringiensis strains. Those cry-type genes included cryIA(a), cryIA(a), cryIA(b), cryIA(b), cryIA(c), cryIB, cryIC, cryIC, cryIC(b), cryID, cryIE, cryIF, cryIF, and cryIII (a dagger at the end of a gene designation indicates a novel cry-type gene determined by restriction mapping or DNA sequences). Among them, the sequences of cryIA(a), cryIA(b), cryIB, cryIC, cryIF, and cryIII were found to be different from the corresponding published cry gene sequences. Interestingly, five cry-type genes [cryIA(a)-, cryIB-, cryIC-, cryIC(b)-, and cryIF-type genes] and seven cry-type genes [cryIA(a)-, cryIA(b)-, cryIB-, cryIC-, cryIC(b)-, cryIF-, and cryIII-type genes] have been detected from B. thuringiensis subsp. morrisoni HD-12 and B. thuringiensis subsp. wuhanensis, respectively. Therefore, the PCR-RFLP typing system is a facile method to detect both known and novel cry genes existing in B. thuringiensis strains.

Specific PCR primers directed to identify cryI and cryIII genes within a Bacillus thuringiensis strain collection

In this paper we describe a PCR strategy that can be used to rapidly identify Bacillus thuringiensis strains that harbor any of the known cryI or cryIII genes. Four general PCR primers which amplify DNA fragments from the known cryI or cryIII genes were selected from conserved regions. Once a strain was identified as an organism that contains a particular type of cry gene, it could be easily characterized by performing additional PCR with specific cryI and cryIII primers selected from variable regions. The method described in this paper can be used to identify the 10 different cryI genes and the five different cryIII genes. One feature of this screening method is that each cry gene is expected to produce a PCR product having a precise molecular weight. The genes which produce PCR products having different sizes probably represent strains that harbor a potentially novel cry gene. Finally, we present evidence that novel crystal genes can be identified by the method described in this paper.

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.

Nucleotide sequence of the gene encoding novel delta-endotoxin from Bacillus thuringiensis serovar japonensis strain Buibui specific to scarabaeid beetles

A new isolate of Bacillus thuringiensis serovar japonensis strain Buibui, which was specific to scarab beetles (M. Ohba et al., Lett. Appl. Microbiol. 14:54, 1992), was shown to have a 130-kDa insecticidal crystal protein (ICP) (H. Hori et al., J. Appl. Bacteriol. 76:307, 1994). ClaI restriction enzyme fragments of total cell DNA of the isolate were cloned into E. coli (Sato et al., Curr. Microbiol. 28:15, 1994). Whole 3480-bp nucleotide sequence of the gene encoding 130-kDa ICP was determined, and the molecular weight of the ICP was estimated to be 130,424. The strongly conserved five blocks that occur in almost all ICP genes of B. thuringiensis were detected in the ORF with the same order and almost the same intervals as elsewhere. The amino acid sequence homologies of the whole ICP or N-terminus half portion to that of the CryIIIA, B, C, D, and CryV were about 35%.

Characterization of larvicidal toxin protein from Bacillus thuringiensis serovar japonensis strain Buibui specific for scarabaeid beetles

The delta-endo toxin proteins from Bacillus thuringiensis which kill the larvae of various scarabaeid beetles such as Anomala cuprea, A. rufocuprea and Popillia japonica were purified by DEAE ion exchange chromatography. A protein with a molecular size of 130 kDa was purified. During the purification a minor peak was also detected which was estimated to be 67 kDa by SDS-PAGE. Both 130 and 67 kDa proteins showed larvicidal activity against A. cuprea. The lethal concentration of the 130 kDa protein which killed 50% of the larvae tested (LC50) against A. cuprea was 2 micrograms g-1 compost. A comparison by SDS-PAGE of the V8 protease digestion pattern of the 130 and 67 kDa larvicidal proteins showed that proteolytic resistant core peptides of approximately 60 kDa molecular size were resulted. The N-terminus amino acid sequence of the 130 and 67 kDa proteins was determined to be NH2-XXPNNQNEYEIIDAL and NH2-XSRNPGTFI, respectively, which is not identical to the sequence of CryIA, CryIB, CryIC and CryIII proteins.

Cloning, heterologous expression, and localization of a novel crystal protein gene from Bacillus thuringiensis serovar japonensis strain buibui toxic to scarabaeid insects

Recombinant Escherichia coli strains harboring pAG1, pAG2, pKBB100, and pKBB101 were cloned by using antiserum constructed against 130-kDa crystal protein antigen of Bacillus thuringiensis serovar japonensis strain Buibui. DNAs in the recombinant strains hybridized to the 26-base oligonucleotide probe corresponding to N-terminal amino acids of the 130-kDa crystal protein of strain Buibui. Cultures of the recombinant strains were toxic to larvae of the cupreous chafer, Anomala cuprea. Furthermore, the production of the 130-kDa polypeptide was demonstrated in the cells harboring pAG1 and pAG2 by immunoblot analysis with antiserum against the 130-kDa crystal protein. Southern hybridization analysis showed that the 130-kDa crystal protein gene is located on the chromosomal DNA of strain Buibui. On the other hand, DNA probes derived from cryIA(a) and cryIIIA genes did not hybridize to the DNA of strain Buibui.

Characterization of two genes encoding Bacillus thuringiensis insecticidal crystal proteins toxic to Coleoptera species

Bacillus thuringiensis EG2838 and EG4961 are highly toxic to Colorado potato beetle larvae, and only strain EG4961 is toxic to southern corn rootworm larvae. To investigate the cause of the different insecticidal activities of EG2838 and EG4961, cryIII-type genes toxic to coleopterans were cloned from each strain. The cryIIIB gene, cloned as part of an 8.0-kb EcoRI fragment of EG2838 DNA, encoded a crystal protein (CryIIIB) of 74,237 Da. The cryIIIB2 gene, cloned as part of an 8.3-kb PstI-Asp718 fragment of EG4961 DNA, encoded a crystal protein (CryIIIB2) of 74,393 Da that was 94% identical to CryIIIB. Analysis of the transcriptional start sites showed that cryIIIB and cryIIIB2 were initiated from a conserved region located within 130 nucleotides upstream from the translation start sites of both genes. Although the CryIIIB and CryIIIB2 proteins were similar in sequence, they displayed distinct insecticidal activities: CryIIIB was one-third as toxic as CryIIIB2 to Colorado potato beetle larvae, and CryIIIB2, but not CryIIIB, was toxic to southern corn rootworm larvae. Genes encoding crystal proteins of approximately 32 and 31 kDa were located adjacent to the cryIIIB and cryIIIB2 genes, respectively. The 32- and 31-kDa crystal proteins failed to enhance the insecticidal activities of CryIIIB and CryIIIB2.

The delta-endotoxin protein family displays a hydrophobic motif that might be implicated in toxicity

A computer-based analysis of hydropathy and surface probability of representative members of each class of the Cry family of proteins was performed. A highly conserved hydrophobic motif within the previously described block, D2, is present not only in lepidopteran toxin genes but also in toxins active against diptera and coleoptera. An interesting feature of this hydrophobic motif is the presence of an aspartic residue (highly hydrophilic) in its middle part. Comparison with the amino acid sequence from diphtheria toxin showed that it also contains a hydrophobic motif similar to the one present in the Bacillus thuringiensis toxins. It also contains an aspartic residue in the middle part and some speculations are presented on the function of this specific region with regard to the toxic mechanism of action.

Novel Bacillus thuringiensis insecticidal crystal protein with a silent activity against coleopteran larvae

A novel Bacillus thuringiensis crystal protein with a silent activity against the Colorado potato beetle is described. The crystal proteins are produced as bipyramidal crystals. These crystals contain a protein of 129 kDa with a trypsin-resistant core fragment of 72 kDa. Neither a spore-crystal mixture nor in vitro-solubilized crystals are toxic to any of several Lepidoptera and Coleoptera species tested. In contrast, a trypsin-treated solution containing the 72-kDa tryptic core fragment of the protoxin is highly toxic to Colorado potato beetle larvae. The crystal protein-encoding gene was cloned and sequenced. The inferred amino acid sequence of the putative toxic fragment has 37, 32, and 33% homology to the CryIIIA, CryIIIB, and CryIIID toxins, respectively. Interestingly, the 501 C-terminal amino acids show 41 to 48% amino acid identity with corresponding C-terminal amino acid sequences of other crystal proteins. Because of the toxicity of the fragment to the Colorado potato beetle and because of the distinct similarities of the toxic fragment with the other CryIII proteins, this gene was given a new subclass name (cryIIIC) within the CryIII class of coleopteran-active crystal proteins. CryIIIC represents the first example of a crystal protein with a silent activity towards coleopteran insect larvae. Natural CryIIIC crystals are not toxic. Toxicity is revealed only after an in vitro solubilization and activation step.

Nucleotide sequence of gene cryIIID encoding a novel coleopteran-active crystal protein from strain BTI109P of Bacillus thuringiensis subsp. kurstaki

The nucleotide sequence of a novel insecticidal crystal protein(Cry)-encoding gene from a Bacillus thuringiensis serotype kurstaki isolate is described. The gene is related to the known coleopteran-active cryIII genes and encodes a CryIIID that is much more active against Colorado potato beetle than other CryIII.

Molecular characterization of two novel crystal protein genes from Bacillus thuringiensis subsp. thompsoni

Two genes encoding the predominant polypeptides of Bacillus thuringiensis subsp. thompsoni cuboidal crystals were cloned in Escherichia coli and sequenced. The polypeptides have electrophoretic mobilities of 40 and 34 kDa, with the deduced amino acid sequences predicting molecular masses of 35,384 and 37,505 Da, respectively. No statistically significant similarities were detected between the 40- or 34-kDa crystal protein and any other characterized B. thuringiensis crystal protein, nor were they detected between the 40- and 34-kDa crystal proteins. A 100-MDa plasmid carries both crystal protein genes, which appear to be part of an operon, with the 40-kDa gene 64 nucleotides upstream of the 34-kDa gene. Both crystal proteins are synthesized in approximately the same amounts. Even though small compared with other crystal proteins, the 34-kDa crystal protein has insecticidal activity against lepidopteran larvae (Manduca sexta). The 40-kDa polypeptide appears to have no insecticidal activity, but it could have a role in crystal structure.

Prediction of insecticidal activity of Bacillus thuringiensis strains by polymerase chain reaction product profiles

A rapid analysis of Bacillus thuringiensis strains predictive of insecticidal activity was established by using polymerase chain reaction (PCR) technology. Primers specific to regions of high homology within genes encoding three major classes of B. thuringiensis crystal proteins were used to generate a PCR product profile characteristic of each insecticidal class. Predictions of insecticidal activity were made on the basis of the electrophoretic patterns of the PCR products. Included in the screen were PCR primers specific for cryI, cryIII, and cryIV genes, which are insecticidal for lepidopterans, coleopterans, and dipterans, respectively. Known B. thuringiensis strains as well as unidentified strains isolated from soil and insect cadavers were analyzed by PCR. Small amounts of crude sample lysates were assayed in a single PCR reaction containing 12 to 20 primers capable of distinguishing between the different insecticidal genes. Insecticidal activity predicted by the PCR screen was found to correspond with the insecticidal activity of insect bioassays. In addition to identifying strains with known insecticidal genes, the PCR screen can identify strains with altered electrophoretic patterns containing potentially novel genes.

Two novel strains of Bacillus thuringiensis toxic to coleopterans

Two novel strains of Bacillus thuringiensis were isolated from native habitats by the use of genes coding for proteins toxic to coleopterans (cryIII genes) as hybridization probes. Strain EG2838 (isolated by the use of the cryIIIA probe) contained a cryIIIA-hybridizing plasmid of approximately 100 MDa and synthesized crystal proteins of approximately 200 (doublet), 74, 70, 32, and 28 kDa. Strain EG4961 (isolated by the use of a cryIIIA-related probe) contained a cryIIIA-hybridizing plasmid of approximately 95 MDa and synthesized crystal proteins of 74, 70, and 30 kDa. Structural relationships among the crystal proteins of strains EG2838 and EG4961 were detected; antibodies to the CryIIIA protein toxic to coleopterans reacted with the 74- and 70-kDa proteins of EG2838 and EG4961, antibodies to the 32-kDa plus 28-kDa proteins of EG2838 reacted with the 30-kDa protein of EG4961, and antibodies to the 200-kDa proteins of EG2838 reacted with the 28-kDa protein of EG2838. Experiments with B. thuringiensis flagella antibody reagents demonstrated that EG2838 belongs to H serotype 9 (reference strain B. thuringiensis subsp. tolworthi) and that EG4961 belongs to H serotype 18 (reference strain B. thuringiensis subsp. kumamotoensis). A mixture of spores plus crystal proteins of either EG2838 or EG4961 was toxic to the larvae of Colorado potato beetle (Leptinotarsa decemlineata), and significantly, the EG4961 mixture was also toxic to the larvae of southern corn rootworm (Diabrotica undecimpunctata howardi). DNA restriction blot analysis suggested that strains EG2838 and EG4961 each contained a unique gene coding for a protein toxic to coleopterans.