A 20-kilodalton protein preserves cell viability and promotes CytA crystal formation during sporulation in Bacillus thuringiensis

The Perpetual Vector Mosquito Threat and Its Eco-Friendly Nemeses

Mosquitoes are the most notorious arthropod vectors of viral and parasitic diseases for which approximately half the world's population, ~4,000,000,000, is at risk. Integrated pest management programs (IPMPs) have achieved some success in mitigating the regional transmission and persistence of these diseases. However, as many vector-borne diseases remain pervasive, it is obvious that IPMP successes have not been absolute in eradicating the threat imposed by mosquitoes. Moreover, the expanding mosquito geographic ranges caused by factors related to climate change and globalization (travel, trade, and migration), and the evolution of resistance to synthetic pesticides, present ongoing challenges to reducing or eliminating the local and global burden of these diseases, especially in economically and medically disadvantaged societies. Abatement strategies include the control of vector populations with synthetic pesticides and eco-friendly technologies. These "green" technologies include SIT, IIT, RIDL, CRISPR/Cas9 gene drive, and biological control that specifically targets the aquatic larval stages of mosquitoes. Regarding the latter, the most effective continues to be the widespread use of Lysinibacillus sphaericus (Ls) and Bacillus thuringiensis subsp. israelensis (Bti). Here, we present a review of the health issues elicited by vector mosquitoes, control strategies, and lastly, focus on the biology of Ls and Bti, with an emphasis on the latter, to which no resistance has been observed in the field.

20-kDa accessory protein (P20) from Bacillus thuringiensis subsp. israelensis ISPC-12: Purification, characterization, solution scattering and structural analysis

The 20-kDa accessory protein (P20) from Bacillus thuringiensis subsp. israelensis (Bti) has been identified as an essential molecular chaperone in the enhancement of Cry11Aa and Cyt1Aa toxins production and their bio-crystallization. Additionally, P20 plays a vital role in suppressing the toxic effect of Cyt toxin on the host bacterium and also enhances insecticidal activity of Cry1Ac protein. Thus, the function of P20 is more specific than that of the chaperones. However, P20 is poorly investigated and insufficiently characterized. In the present study, we recombinantly expressed p20 from local isolate Bti ISPC-12 in heterologous bacterium E. coli and P20 protein was purified to homogeneity. Detailed biochemical and biophysical characterization provides crucial insights about in-vitro behavior as well as spatial conformations of P20 protein. Further, structural modelling and analysis provides insights into three-dimensional organization of the protein and shows that P20 is a non-toxic member of cytolytic (Cyt) toxin family similar to Cyt1Ca, with presence of conserved cytolysin fold. Additionally, solution scattering reveals that P20 is present as a dimer in the solution and probable dimeric assembly of P20 is presented. The findings reported here reveal engaging facts about P20 thereby advancing our understanding about this protein, which will expedite future studies.

The Parasporal Body of Bacillus thuringiensis subsp. israelensis: A Unique Phage Capsid-Associated Prokaryotic Insecticidal Organelle

The three most important commercial bacterial insecticides are all derived from subspecies of Bacillus thuringiensis (Bt). Specifically, Bt subsp. kurstaki (Btk) and Bt subsp. aizawai (Bta) are used to control larval lepidopteran pests. The third, Bt subsp. israelensis (Bti), is primarily used to control mosquito and blackfly larvae. All three subspecies produce a parasporal body (PB) during sporulation. The PB is composed of insecticidal proteins that damage the midgut epithelium, initiating a complex process that results in the death of the insect. Among these three subspecies of Bt, Bti is unique as it produces the most complex PB consisting of three compartments. Each compartment is bound by a multilaminar fibrous matrix (MFM). Two compartments contain one protein each, Cry11Aa1 and Cyt1Aa1, while the third contains two, Cry4Aa1/Cry4Ba1. Each compartment is packaged independently before coalescing into the mature spherical PB held together by additional layers of the MFM. This distinctive packaging process is unparalleled among known bacterial organelles, although the underlying molecular biology is yet to be determined. Here, we present structural and molecular evidence that the MFM has a hexagonal pattern to which Bti proteins Bt152 and Bt075 bind. Bt152 binds to a defined spot on the MFM during the development of each compartment, yet its function remains unknown. Bt075 appears to be derived from a bacteriophage major capsid protein (MCP), and though its sequence has markedly diverged, it shares striking 3-D structural similarity to the Escherichia coli phage HK97 Head 1 capsid protein. Both proteins are encoded on Bti's pBtoxis plasmid. Additionally, we have also identified a six-amino acid motif that appears to be part of a novel molecular process responsible for targeting the Cry and Cyt proteins to their cytoplasmic compartments. This paper describes several previously unknown features of the Bti organelle, representing a first step to understanding the biology of a unique process of sorting and packaging of proteins into PBs. The insights from this research suggest a potential for future applications in nanotechnology.

Bacillus thuringiensis Cyt Proteins as Enablers of Activity of Cry and Tpp Toxins against Aedes albopictus

Aedes albopictus is a species of mosquito, originally from Southeast Asia, that belongs to the Culicidae family and the Dipteran insect order. The distribution of this vector has rapidly changed over the past decade, making most of the temperate territories in the world vulnerable to important human vector-borne diseases such as dengue, yellow fever, zika or chikungunya. Bacillus thuringiensis var. israeliensis (Bti)-based insecticides represent a realistic alternative to the most common synthetic insecticides for the control of mosquito larvae. However, several studies have revealed emerging resistances to the major Bti Crystal proteins such as Cry4Aa, Cry4Ba and Cry11Aa, making the finding of new toxins necessary to diminish the exposure to the same toxicity factors overtime. Here, we characterized the individual activity of Cyt1Aa, Cry4Aa, Cry4Ba and Cry11Aa against A. albopictus and found a new protein, Cyt1A-like, that increases the activity of Cry11Aa more than 20-fold. Additionally, we demonstrated that Cyt1A-like facilitates the activity three new Bti toxins: Cry53-like, Cry56A-like and Tpp36-like. All in all, these results provide alternatives to the currently available Bti products for the control of mosquito populations and position Cyt proteins as enablers of activity for otherwise non-active crystal proteins.

Identification and characterization of a new cry-like gene found in a Bacillus cereus strain

Bacillus thuringiensis is the most successful microbial insecticide against different pests in agriculture and vectors of diseases. Its activity is mostly attributed to the Cry proteins expressed during its sporulation phase. However, these proteins are not exclusive to B. thuringiensis. Some cry genes have been found in other Bacillus species, or even in other genera. In this work, cry genes were searched in 223 acrystalliferous bacillaceous strains. From these strains 13 amplicons were obtained, cloned, and sequenced; however, only 6 amplicons tested positive for cry-like genes, and the 6 isolates showed to be the same strain. We report the characterization of an unusual strain of B. cereus (LBIC-004) which is unable to form protein inclusions during the sporulation phase. LBIC-004 showed a high identity to B. cereus using the sequences of 16S rRNA, gyrB and hag genes; in addition, a unique plasmid pattern of the strain was obtained. A 1953-bp cry gene was identified, coding for a 651 amino acid protein with a molecular weight of 74.9 kDa. This protein showed a predicted three-domain structure, similar to all Cry proteins. However, the amino acid sequence of the protein showed only 41% identity its highest hit: the Cry8Ca1 protein, indicating the uniqueness of this cry-like gene. It was cloned and transferred into a mutant acrystalliferous B. thuringiensis strain which was used in bioassays against Caenorhabditis elegans, Aedes aegypti, Manduca sexta and Phyllophaga sp. The recombinant strain showed no crystal formation and no toxicity to the tested species.

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.

In vivo nanoscale analysis of the dynamic synergistic interaction of Bacillus thuringiensis Cry11Aa and Cyt1Aa toxins in Aedes aegypti

The insecticidal Cry11Aa and Cyt1Aa proteins are produced by Bacillus thuringiensis as crystal inclusions. They work synergistically inducing high toxicity against mosquito larvae. It was proposed that these crystal inclusions are rapidly solubilized and activated in the gut lumen, followed by pore formation in midgut cells killing the larvae. In addition, Cyt1Aa functions as a Cry11Aa binding receptor, inducing Cry11Aa oligomerization and membrane insertion. Here, we used fluorescent labeled crystals, protoxins or activated toxins for in vivo localization at nano-scale resolution. We show that after larvae were fed solubilized proteins, these proteins were not accumulated inside the gut and larvae were not killed. In contrast, if larvae were fed soluble non-toxic mutant proteins, these proteins were found inside the gut bound to gut-microvilli. Only feeding with crystal inclusions resulted in high larval mortality, suggesting that they have a role for an optimal intoxication process. At the macroscopic level, Cry11Aa completely degraded the gastric caeca structure and, in the presence of Cyt1Aa, this effect was observed at lower toxin-concentrations and at shorter periods. The labeled Cry11Aa crystal protein, after midgut processing, binds to the gastric caeca and posterior midgut regions, and also to anterior and medium regions where it is internalized in ordered "net like" structures, leading finally to cell break down. During synergism both Cry11Aa and Cyt1Aa toxins showed a dynamic layered array at the surface of apical microvilli, where Cry11Aa is localized in the lower layer closer to the cell cytoplasm, and Cyt1Aa is layered over Cry11Aa. This array depends on the pore formation activity of Cry11Aa, since the non-toxic mutant Cry11Aa-E97A, which is unable to oligomerize, inverted this array. Internalization of Cry11Aa was also observed during synergism. These data indicate that the mechanism of action of Cry11Aa is more complex than previously anticipated, and may involve additional steps besides pore-formation activity.

Insecticidal Activity of a Cry1Ca toxin of Bacillus thuringiensis Berliner (Firmicutes: Bacillaceae) and Its Synergism with the Cyt1Aa Toxin Against Aedes aegypti (Diptera: Culicidae)

The Cry1C protein family of Bacillus thuringiensis form bipyramidal crystals, which are commonly associated with toxic activity against lepidopteran species; however, some members of this family may also be toxic to dipterans. In the present work, the Cry1Ca16 protein, synthesized by the B. thuringiensis LBIT-1217 strain, was analyzed. The gene coding for this protein was amplified, sequenced, and cloned into the pSTAB vector, which was electro-transferred into the acrystalliferous B. thuringiensis 4Q7 strain. The recombinant strain showed the expected bipyramidal crystal morphology, identical to the original LBIT-1217 strain and exhibited toxicity against larvae of Aedes aegypti (Diptera). Pure crystals from the recombinant strain were used in bioassays against Ae. aegypti larvae, estimating an LC50 of 4.61 μg/ml. Further studies on Cry1Ca16 mosquitocidal potential included joint-action tests with the Cyt1Aa protein crystals from B. thuringiensis israelensis. An LC50 using pure Cyt1Aa crystals was estimated at 0.73 μg/ml, whereas an LC50 of 0.61 μg/ml was estimated when both toxins were tested together. Data from these bioassays was analyzed using joint-action tests such as the Tammes-Bakuniak graphical method and the formula proposed by Tabashnik (1992). Both tests clearly showed a synergistic effect between these two toxins.

Bacillus subtilis as a host for mosquitocidal toxins production

Aedes albopictus transmits several arboviral infections. In the absence of vaccines, control of mosquito populations is the only strategy to prevent vector-borne diseases. As part of the search for novel, biological and environmentally friendly strategies for vector control, the isolation of new bacterial species with mosquitocidal activity represents a promising approach. However, new bacterial isolates may be difficult to grow and genetically manipulate. To overcome these limits, here we set up a system allowing the expression of mosquitocidal bacterial toxins in the well-known genetic background of Bacillus subtilis. As a proof of this concept, the ability of B. subtilis to express individual or combinations of toxins of Bacillus thuringiensis israelensis (Bti) was studied. Different expression systems in which toxin gene expression was driven by IPTG-inducible, auto-inducible or toxin gene-specific promoters were developed. The larvicidal activity of the resulting B. subtilis strains against second-instar Ae. albopictus larvae allowed studying the activity of individual toxins or the synergistic interaction among Cry and Cyt toxins. The expression systems here presented lay the foundation for a better improved system to be used in the future to characterize the larvicidal activity of toxin genes from new environmental isolates.

Serial femtosecond crystallography on in vivo-grown crystals drives elucidation of mosquitocidal Cyt1Aa bioactivation cascade

Cyt1Aa is the one of four crystalline protoxins produced by mosquitocidal bacterium Bacillus thuringiensis israelensis (Bti) that has been shown to delay the evolution of insect resistance in the field. Limiting our understanding of Bti efficacy and the path to improved toxicity and spectrum has been ignorance of how Cyt1Aa crystallizes in vivo and of its mechanism of toxicity. Here, we use serial femtosecond crystallography to determine the Cyt1Aa protoxin structure from sub-micron-sized crystals produced in Bti. Structures determined under various pH/redox conditions illuminate the role played by previously uncharacterized disulfide-bridge and domain-swapped interfaces from crystal formation in Bti to dissolution in the larval mosquito midgut. Biochemical, toxicological and biophysical methods enable the deconvolution of key steps in the Cyt1Aa bioactivation cascade. We additionally show that the size, shape, production yield, pH sensitivity and toxicity of Cyt1Aa crystals grown in Bti can be controlled by single atom substitution.

Bacillus thuringiensis israelensis (Bti) produces the naturally-crystalline proteinaceous toxin Cyt1Aa that is toxic to mosquito larvae. Here the authors grow recombinant nanocrystals of the Cyt1Aa protoxin in vivo and use serial femtosecond crystallography to determine its structure at different redox and pH conditions and by combining their structural data with further biochemical, toxicological and biophysical analyses provide mechanistic insights into the Cyt1Aa bioactivation cascade.

Small RNA-mediated Cry toxin silencing allows Bacillus thuringiensis to evade Caenorhabditis elegans avoidance behavioral defenses

Pathogen avoidance behavior protects animal hosts against microbial pathogens. Pathogens have evolved specific strategies during coevolution in response to such host defenses. However, these strategies for combatting host avoidance behavioral defenses remain poorly understood. Here, we used Caenorhabditis elegans and its bacterial pathogen Bacillus thuringiensis as a model and determined that small RNA (sRNA)-mediated Cry toxin silencing allowed pathogens to evade host avoidance behavioral defenses. The B. thuringiensis strain YBT-1518, which encodes three nematicidal cry genes, is highly toxic to C. elegans. However, the expression of the most potent toxin, Cry5Ba, was silenced in this strain when YBT-1518 was outside the host. Cry5Ba silencing was due to the sRNA BtsR1, which bound to the RBS site of the cry5Ba transcript via direct base pairing and inhibited Cry5Ba expression. Upon ingestion by C. elegans, Cry5Ba was expressed in vivo by strain YBT-1518. Cry5Ba silencing may allow B. thuringiensis to avoid nematode behavioral defenses and then express toxins once ingested to kill the host and gain a survival advantage. Our work describes a novel model of sRNA-mediated regulation to aid pathogens in combating host avoidance behavioral defenses.

The combinatory effect of Cyt1Aa flexibility and specificity against dipteran larvae improves the toxicity of Bacillus thuringensis kurstaki toxins

Cyt1A98 is a novel cytolytic protein, from BUPM98 Bacillus thuringiensis strain, characterized by its synergistic activity with B. thuringiensis kurstaki toxins against lepidopteran larvae. In this study, we evidenced that Cyt1A98 improves the toxicity of B. thuringiensis kurstaki toxins against Aedes aegypti larvae. In fact, the strain BNS3pHTcyt1A98 exhibited a larvicidal activity of about 849-fold of that of BNS3pHTBlue against A. aegypti. The molecular and biochemical characterizations, of cyt1A98 gene and its product, were achieved. Cyt1A98 had an LC₅₀ value of about 126.56 mg l^-1 against A. aegypti larvae. Compared to Cyt1Aa of B. thuringiensis israelensis, Cyt1A98 amino acid sequence harbours three substitutions of three conserved amino acids among Cyt1Aa family members (Ser42Pro, Pro82Ala, Met188Thr). The Cyt1A98 protein structural analysis evidenced more flexibility than Cyt1Aa. According to the high fluctuation observed for the residue Pro42, the amino acid at position 42 is implicated in the flexibility property of Cyt1Aa especially for the αC and αD helices, involved in the penetration into the cell membrane. The toxicity improvement could be probably due to the higher flexibility combined with the specific affinity toward dipteran larvae. The Cyt1A/B. thuringiensis kurstaki Cry toxins model provides a potential molecular genetic strategy for an efficient bioinsecticide.

Highly Effective Broad Spectrum Chimeric Larvicide That Targets Vector Mosquitoes Using a Lipophilic Protein

Two mosquitocidal bacteria, Bacillus thuringiensis subsp. israelensis (Bti) and Lysinibacillus sphaericus (Ls) are the active ingredients of commercial larvicides used widely to control vector mosquitoes. Bti’s efficacy is due to synergistic interactions among four proteins, Cry4Aa, Cry4Ba, Cry11Aa, and Cyt1Aa, whereas Ls’s activity is caused by Bin, a heterodimer consisting of BinA, the toxin, and BinB, a midgut-binding protein. Cyt1Aa is lipophilic and synergizes Bti Cry proteins by increasing midgut binding. We fused Bti’s Cyt1Aa to Ls’s BinA yielding a broad-spectrum chimeric protein highly mosquitocidal to important vector species including Anopheles gambiae, Culex quinquefasciatus, and Aedes aegypti, the latter an important Zika and Dengue virus vector insensitive to Ls Bin. Aside from its vector control potential, our bioassay data, in contrast to numerous other reports, provide strong evidence that BinA does not require conformational interactions with BinB or microvillar membrane lipids to bind to its intracellular target and kill mosquitoes.

Contributions of 5'-UTR and 3'-UTR cis elements to Cyt1Aa synthesis in Bacillus thuringiensis subsp. israelensis

The biopesticide used most effectively to control mosquito and blackfly vectors of human diseases worldwide is Bacillus thuringiensis subsp. israelensis. The high efficacy of this bacterium is due to synergistic interactions among four protein entomotoxins assembled individually into a single parasporal body (PB) during sporulation. Cyt1Aa, the primary synergist, is the most abundant toxin, comprising approximately 55% of the PB's mass. The other proteins are Cry11Aa at ∼35%, and Cry4Aa and Cry4Ba, which together account for the remaining ∼10%. The molecular genetic basis for the comparatively large amount of Cyt1Aa synthesized is unknown. Here, in addition to the known strong BtI (σ^E) and BtII (σ^K) promoters, we demonstrate a third promoter (BtIII) that has high identity to the σ^E promoter of Bacillus subtilis, contributes to the large amount of Cyt1Aa synthesized. We also show that a cyt1Aa-BtIII construct was not functional in a σ^E-deficient strain of B. subtilis. Comparison of transcription levels and protein profiles for recombinant strains containing different combinations of BtI, BtII and BtIII, or each promoter alone, showed that BtIII is active throughout sporulation. We further demonstrate that a stable stem-loop in the 3'-untranslated region (3'-UTR, predicted ΔG=-27.6) contributes to the high level of Cyt1Aa synthesized.

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.

Toxin stability improvement and toxicity increase against dipteran and lepidopteran larvae of Bacillus thuringiensis crystal protein Cry2Aa

BACKGROUND

Bacillus thuringiensis δ-endotoxins are the most widely used biopesticides for controlling economically important crop pests and disease vectors. Improving their efficacy is of great benefit. Here, an improvement in Cry2Aa δ-endotoxin toxicity was attempted via a cry gene over expression system using P20 from B. thuringiensis israelensis.

RESULTS

The coexpression of Cry2Aa with P20 resulted in a seven fold increase in its production yield in B. thuringiensis. Generated crystals proved to be significantly more toxic (505.207 µg g^-1 , 1.99 mg L^-1 and 1.49 mg L^-1 ) than the P20-lacking control (720.78 µg g^-1 , 705.69 mg L^-1 and 508.51 mg L^-1 ) against Ephestia kuehniella, Aedes aegypti and Culex pipiens larvae respectively. In vitro, processing experiments revealed a P20-mediated protection of Cry2Aa against degradation under larval gut conditions. Thus, P20 could promote the maintenance of a tightly packaged conformation of Cry2Aa toxins in the larval midgut upon correct activation and binding to its membrane receptors.

CONCLUSION

Based on their resistance against excessive proteolysis, Cry2Aa δ-endotoxins, produced in the presence of P20, could be considered as a successful control agent for E. kuehniella and an effective alternative for mosquito control, implying its possible exploitation in pest management programmes. © 2016 Society of Chemical Industry.

De novo phasing with X-ray laser reveals mosquito larvicide BinAB structure

BinAB is a naturally occurring paracrystalline larvicide distributed worldwide to combat the devastating diseases borne by mosquitoes. These crystals are composed of homologous molecules, BinA and BinB, which play distinct roles in the multi-step intoxication process, transforming from harmless, robust crystals, to soluble protoxin heterodimers, to internalized mature toxin, and finally toxic oligomeric pores. The small size of the crystals, 50 unit cells per edge, on average, has impeded structural characterization by conventional means. Here, we report the structure of BinAB solved de novo by serial-femtosecond crystallography at an X-ray free-electron laser (XFEL). The structure reveals tyrosine and carboxylate-mediated contacts acting as pH switches to release soluble protoxin in the alkaline larval midgut. An enormous heterodimeric interface appears responsible for anchoring BinA to receptor-bound BinB for co-internalization. Remarkably, this interface is largely composed of propeptides, suggesting that proteolytic maturation would trigger dissociation of the heterodimer and progression to pore formation.

The use of structural modelling to infer structure and function in biocontrol agents

Homology modelling can provide important insights into the structures of proteins when a related protein structure has already been solved. However, for many proteins, including a number of invertebrate-active toxins and accessory proteins, no such templates exist. In these cases, techniques of ab initio, template-independent modelling can be employed to generate models that may give insight into structure and function. In this overview, examples of both the problems and the potential benefits of ab initio techniques are illustrated. Consistent modelling results may indicate useful approximations to actual protein structures and can thus allow the generation of hypotheses regarding activity that can be tested experimentally.

Improvement and efficient display of Bacillus thuringiensis toxins on M13 phages and ribosomes

Bacillus thuringiensis (Bt) produces insecticidal proteins that have been used worldwide in the control of insect-pests in crops and vectors of human diseases. However, different insect species are poorly controlled by the available Bt toxins or have evolved resistance to these toxins. Evolution of Bt toxicity could provide novel toxins to control insect pests. To this aim, efficient display systems to select toxins with increased binding to insect membranes or midgut proteins involved in toxicity are likely to be helpful. Here we describe two display systems, phage display and ribosome display, that allow the efficient display of two non-structurally related Bt toxins, Cry1Ac and Cyt1Aa. Improved display of Cry1Ac and Cyt1Aa on M13 phages was achieved by changing the commonly used peptide leader sequence of the coat pIII-fusion protein, that relies on the Sec translocation pathway, for a peptide leader sequence that relies on the signal recognition particle pathway (SRP) and by using a modified M13 helper phage (Phaberge) that has an amber mutation in its pIII genomic sequence and preferentially assembles using the pIII-fusion protein. Also, both Cry1Ac and Cyt1Aa were efficiently displayed on ribosomes, which could allow the construction of large libraries of variants. Furthermore, Cry1Ac or Cyt1Aa displayed on M13 phages or ribosomes were specifically selected from a mixture of both toxins depending on which antigen was immobilized for binding selection. These improved systems may allow the selection of Cry toxin variants with improved insecticidal activities that could counter insect resistances.

Evolution of Resistance in Culex quinquefasciatus (Say) Selected With a Recombinant Bacillus thuringiensis Strain-Producing Cyt1Aa and Cry11Ba, and the Binary Toxin, Bin, From Lysinibacillus sphaericus

Fourth instars of Culex quinquefasciatus (Say) (Diptera: Culicidae) were selected with a recombinant bacterial strain synthesizing the mosquitocidal proteins from Lysinibacillus sphaericus (Bin) and Cry11Ba and Cyt1Aa from Bacillus thuringiensis. Selection was initiated in Generation 1 with a concentration of 0.04 μg/ml, which rose to a maximum selection concentration of 8.0 μg/ml in Generation 14, followed by an unexpected, rapid increase in mortality in Generation 15. Subsequently, a selection concentration of 0.8 μg/ml was determined to be survivable. During this same period, resistance rose to nearly 1,000-fold (by Generation 12) and declined to 18.8-fold in Generation 19. Resistance remained low and fluctuated between 5.3 and 7.3 up to Generation 66. The cross-resistance patterns and interactions among the component proteins were analyzed to identify possible causes of this unusual pattern of evolution. Poor activity in the mid-range concentrations and lower-than-expected synergistic interactions were identified as potential sources of the early resistance. These findings should be considered in the development of genetically engineered strains intended to control nuisance and vector mosquitoes.

The expression and crystallization of Cry65Aa require two C-termini, revealing a novel evolutionary strategy of Bacillus thuringiensis Cry proteins

The insecticidal crystal protein (Cry) genes of Bacillus thuringiensis are a key gene resource for generating transgenic crops with pest resistance. However, many cry genes cannot be expressed or form crystals in mother cells. Here, we report a novel Cry protein gene, cry65Aa1, which exists in an operon that contains a downstream gene encoding a hypothetical protein ORF2. We demonstrated that ORF2 is required for Cry65Aa1 expression and crystallization by function as a C-terminal crystallization domain. The orf2 sequence is also required for Cry65Aa expression, because orf2 transcripts have a stabilizing effect on cry65Aa1 transcripts. Furthermore, we found that the crystallization of Cry65Aa1 required the Cry65Aa1 C-terminus in addition to ORF2 or a typical Cry protein C-terminal region. Finally, we showed that Cry65Aa1 has a selective cytotoxic effect on MDA-MB231 cancer cells. This report is the first description of a 130-kDa mass range Cry protein requiring two C-termini for crystallization. Our findings reveal a novel evolutionary strategy of Cry proteins and provide an explanation for the existence of Cry protein genes that cannot form crystals in B. thuringiensis. This study also provides a potential framework for isolating novel cry genes from “no crystal” B. thuringiensis strains.

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.

Regulation of cry gene expression in Bacillus thuringiensis

Bacillus thuringiensis differs from the closely related Bacillus cereus group species by its ability to produce crystalline inclusions. The production of these crystals mainly results from the expression of the cry genes, from the stability of their transcripts and from the synthesis, accumulation and crystallization of large amounts of insecticidal Cry proteins. This process normally coincides with sporulation and is regulated by various factors operating at the transcriptional, post-transcriptional, metabolic and post-translational levels.

The cry toxin operon of Clostridium bifermentans subsp. malaysia is highly toxic to Aedes Larval Mosquitoes

The management and control of mosquito vectors of human disease currently rely primarily on chemical insecticides. However, larvicidal treatments can be effective, and if based on biological insecticides, they can also ameliorate the risk posed to human health by chemical insecticides. The aerobic bacteria Bacillus thuringiensis and Lysinibacillus sphaericus have been used for vector control for a number of decades. But a more cost-effective use would be an anaerobic bacterium because of the ease with which these can be cultured. More recently, the anaerobic bacterium Clostridium bifermentans subsp. malaysia has been reported to have high mosquitocidal activity, and a number of proteins were identified as potentially mosquitocidal. However, the cloned proteins showed no mosquitocidal activity. We show here that four toxins encoded by the Cry operon, Cry16A, Cry17A, Cbm17.1, and Cbm17.2, are all required for toxicity, and these toxins collectively show remarkable selectivity for Aedes rather than Anopheles mosquitoes, even though C. bifermentans subsp. malaysia is more toxic to Anopheles. Hence, toxins that target Anopheles are different from those expressed by the Cry operon.

Early transcription of Bacillus thuringiensis cry genes in strains active on Lepidopteran species and the role of gene content on their expression

Six strains of Bacillus thuringiensis previously selected as highly toxic against Manduca sexta and Plutella xylostella were analyzed by PCR screening in order to identify the cry genes active on Lepidoptera. According to their gene content and insecticidal potency, these strains were cultured and aliquots taken at different pre- and post-sporulation times. Total RNA was extracted and used as template in RT-PCR analyses directed to identify mRNAs of the previously identified cry genes. Results showed transcription of genes cry1A, cry1E, cry1I, and cry2 even before the onset of sporulation. However, this early transcription did not lead to an appreciable parasporal protein synthesis until t5-t9, as deduced from SDS-PAGE profiles. As for cry1I gene, the corresponding protein was not detected, as expected, but cry1I mRNAs were present at least until t5. Interestingly, strains expressing four cry genes from the end of the log phase onwards exhibited kinetics characterized by a very long transition phase, whereas the strain expressing only one cry gene showed a very short transition phase. Strains expressing three genes showed an intermediate profile. These results indicate that the transcription of B. thuringiensis cry1 and cry2 genes in natural strains can start several hours before massive crystal synthesis occurs and that this translation is probably competing with transcriptional regulators required for the sporulation onset.

Bacillus thuringiensis subsp. israelensis and its dipteran-specific toxins

Bacillus thuringiensis subsp. israelensis (Bti) is the first Bacillus thuringiensis to be found and used as an effective biological control agent against larvae of many mosquito and black fly species around the world. Its larvicidal activity resides in four major (of 134, 128, 72 and 27 kDa) and at least two minor (of 78 and 29 kDa) polypeptides encoded respectively by cry4Aa, cry4Ba, cry11Aa, cyt1Aa, cry10Aa and cyt2Ba, all mapped on the 128 kb plasmid known as pBtoxis. These six δ-endotoxins form a complex parasporal crystalline body with remarkably high, specific and different toxicities to Aedes, Culex and Anopheles larvae. Cry toxins are composed of three domains (perforating domain I and receptor binding II and III) and create cation-selective channels, whereas Cyts are composed of one domain that acts as well as a detergent-like membrane perforator. Despite the low toxicities of Cyt1Aa and Cyt2Ba alone against exposed larvae, they are highly synergistic with the Cry toxins and hence their combinations prevent emergence of resistance in the targets. The lack of significant levels of resistance in field mosquito populations treated for decades with Bti-bioinsecticide suggests that this bacterium will be an effective biocontrol agent for years to come.

Bacillus thuringiensis subsp. kurstaki HD1 as a factory to synthesize alkali-labile ChiA74∆sp chitinase inclusions, Cry crystals and spores for applied use

Background

The endochitinase ChiA74 is a soluble secreted enzyme produced by Bacillus thuringiensis that synergizes the entomotoxigenecity of Cry proteins that accumulate as intracellular crystalline inclusion during sporulation. The purpose of this study was to produce alkaline-soluble ChiA74∆sp inclusions in B. thuringiensis, and to determine its effect on Cry crystal production, sporulation and toxicity to an important agronomical insect, Manduca sexta. To this end we deleted the secretion signal peptide-coding sequence of chiA74 (i.e. chiA74∆sp) and expressed it under its native promoter (pEHchiA74∆sp) or strong chimeric sporulation-dependent cytA-p/STAB-SD promoter (pEBchiA74∆sp) in Escherichia coli, acrystalliferous B. thuringiensis (4Q7) and B. thuringiensis HD1.

Results

Based on mRNA analyses, up to ~9-fold increase in expression of chiA74∆sp was observed using the cytA-p/STAB-SD promoter. ChiA74∆sp (~70 kDa) formed intracellular inclusions that frequently accumulated at the poles of cells. ChiA74∆sp inclusions were dissolved in alkali and reducing conditions, similar to Cry crystals, and retained its activity in a wide range of pH (5 to 9), but showed a drastic reduction (~70%) at pH 10. Chitinase activity of E. coli-pEHchiA74∆sp was ~150 mU/mL, and in E. coli-pEBchiA74∆sp, 250 mU/mL. 4Q7-pEBchiA74∆sp and 4Q7-pEHchiA74∆sp had activities of ~127 mU/mL and ~41 mU/mL, respectively. The endochitinase activity in HD1-pEBchiA74∆sp increased 42x when compared to parental HD1 strain. HD1-pEBchiA74∆sp and HD1 harbored typical bipyramidal Cry inclusions, but crystals in the recombinant were ~30% smaller. Additionally, a 3x increase in the number of viable spores was observed in cultures of the recombinant strain when compared to HD1. Bioassays against first instar larvae of M. sexta with spore-crystals of HD1 or spore-crystal-ChiA74∆sp inclusions of HD1-pEBchiA74∆sp showed LC₅₀s of 67.30 ng/cm² and 41.45 ng/cm², respectively.

Conclusions

Alkali-labile ChiA74∆sp inclusion bodies can be synthesized in E. coli and B. thuringiensis strains. We demonstrated for the first time the applied utility of synthesis of ChiA74∆sp inclusions, Cry crystals and spores in the same sporangium of HD1, a strain used successfully worldwide to control economically significant lepidopteran pests of agriculture. Our findings will allow to us develop strategies to modify expression of ChiA74∆sp while maximizing Cry crystal synthesis in commercial strains of B. thuringiensis.

The 60-kilodalton protein encoded by orf2 in the cry19A operon of Bacillus thuringiensis subsp. jegathesan functions like a C-terminal crystallization domain

The cry19A operon of Bacillus thuringiensis subsp. jegathesan encodes two proteins, mosquitocidal Cry19A (ORF1; 75 kDa) and an ORF2 (60 kDa) of unknown function. Expression of the cry19A operon in an acrystalliferous strain of B. thuringiensis (4Q7) yielded one small crystal per cell, whereas no crystals were produced when cry19A or orf2 was expressed alone. To determine the function of the ORF2 protein, different combinations of Cry19A, ORF2, and the N- or C-terminal half of Cry1C were synthesized in strain 4Q7. Stable crystalline inclusions of these fusion proteins similar in shape to those in the strain harboring the wild-type operon were observed in sporulating cells. Comparative analysis showed that ORF2 shares considerable amino acid sequence identity with the C-terminal region of large Cry proteins. Together, these results suggest that ORF2 assists in synthesis and crystallization of Cry19A by functioning like the C-terminal domain characteristic of Cry protein in the 130-kDa mass range. In addition, to determine whether overexpression of the cry19A operon stabilized its shape and increased Cry19A yield, it was expressed under the control of the strong chimeric cyt1A-p/STAB-SD promoter. Interestingly, in contrast to the expression seen with the native promoter, overexpression of the operon yielded uniform bipyramidal crystals that were 4-fold larger on average than the wild-type crystal. In bioassays using the 4th instar larvae of Culex quinquefasciatus, the strain producing the larger Cry19A crystal showed moderate larvicidal activity that was 4-fold (95% lethal concentration [LC(95)] = 1.9 μg/ml) more toxic than the activity produced in the strain harboring the wild-type operon (LC(95) = 8.2 μg/ml).

The 20-kDa chaperone-like protein of Bacillus thuringiensis ssp. israelensis enhances yield, crystal size and solubility of Cry3A

AIMS

  To determine whether the 20-kDa chaperone-like protein of Bacillus thuringiensis ssp. israelensis enhances synthesis, crystallization and solubility of the Cry3A coleopteran toxin and whether the crystalline inclusions produced are toxic to neonates of the Colorado potato beetle, Leptinotarsa decemlineata.

METHODS AND RESULTS

  The cry3A gene was expressed in the 4Q7 strain of B. thuringiensis ssp. israelensis in the absence or presence of the 20-kDa gene. The 20-kDa protein enhanced Cry3A yield by 2·7-fold per unit of fermentation medium. Crystal volumes averaged 2·123 and 0·964 μm(3) when synthesized in, respectively, the presence or absence of the 20-kDa protein. Both crystals were soluble at pH 5 and pH 6; however, the larger crystal was 1·7× and 1·5× more soluble at, respectively, pH 7 and pH 10. No significant difference in toxicity against L. decemlineata neonates was observed.

CONCLUSIONS

  This report demonstrated that the 20-kDa chaperone-like protein enhances yield, volume and solubility of the coleopteran Cry3A crystalline inclusions per unit crystal/spore mixture.

SIGNIFICANCE AND IMPACT OF THE STUDY

  This is the first report showing that an accessory protein (20-kDa) could enhance synthesis and crystallization of Cry3A, a finding that could be beneficial for commercial production of this coleopteran-specific insecticidal protein for microbial insecticides and possibly even for transgenic crops.

A 54-kilodalton protein encoded by pBtoxis is required for parasporal body structural integrity in Bacillus thuringiensis subsp. israelensis

Strains of Bacillus thuringiensis such as B. thuringiensis subsp. israelensis (ONR-60A) and B. thuringiensis subsp. morrisoni (PG-14) pathogenic for mosquito larvae produce a complex parasporal body consisting of several protein endotoxins synthesized during sporulation that form an aggregate of crystalline inclusions bound together by a multilamellar fibrous matrix. Most studies of these strains focus on the molecular biology of the endotoxins, and although it is known that parasporal body structural integrity is important to achieving high toxicity, virtually nothing is known about the matrix that binds the toxin inclusions together. In the present study, we undertook a proteomic analysis of this matrix to identify proteins that potentially mediate assembly and stability of the parasporal body. In addition to fragments of their known major toxins, namely, Cry4Aa, Cry4Ba, Cry11Aa, and Cyt1Aa, we identified peptides with 100% identity to regions of Bt152, a protein coded for by pBtoxis of B. thuringiensis subsp. israelensis, the plasmid that encodes all endotoxins of this subspecies. As it is known that the Bt152 gene is expressed in B. thuringiensis subsp. israelensis, we disrupted its function and showed that inactivation destabilized the parasporal body matrix and, concomitantly, inclusion aggregation. Using fluorescence microscopy, we further demonstrate that Bt152 localizes to the parasporal body in both strains, is absent in other structural or soluble components of the cell, including the endospore and cytoplasm, and in ligand blots binds to purified multilamellar fibrous matrix. Together, the data show that Bt152 is essential for stability of the parasporal body of these strains.

Persistence of Bacillus thuringiensis israelensis (Bti) in the environment induces resistance to multiple Bti toxins in mosquitoes

BACKGROUND

The simultaneous production of six different toxins by Bacillus thuringiensis israelensis (Bti) is thought to delay the evolution of resistance in treated mosquito populations. Recent studies have shown that Bti can persist and proliferate in the environment, thereby imposing continuous selective pressure on mosquito populations, raising concerns about the long-term effectiveness of this bioinsecticide. In order to evaluate the effect of Bti persistence on the evolution of resistance, the authors selected a laboratory Aedes aegypti L. strain with field-collected leaf litter containing Bti toxins.

RESULTS

It is shown that resistance to each individual Bti toxin (up to 30-fold) can be obtained after only a few generations of selection. However, the resistance to commercial Bti and to environmental Bti remains low (twofold and 3.4-fold respectively) in the selected strain. Furthermore, some selected individuals exhibited resistance to Cry4B but not to Cry4A, suggesting that two distinct resistance mechanisms are involved in the resistance to these two toxins.

CONCLUSION

Considering that resistance to Cry toxins might act as a first step to resistance to a complete Bti toxin mixture, the present results highlight the importance of testing each toxin individually in order accurately to monitor Bti toxin resistance evolution in field populations.

The amino- and carboxyl-terminal fragments of the Bacillus thuringensis Cyt1Aa toxin have differential roles in toxin oligomerization and pore formation

The Cyt toxins produced by the bacteria Bacillus thuringiensis show insecticidal activity against some insects, mainly dipteran larvae, being able to kill mosquitoes and black flies. However, they also possess a general cytolytic activity in vitro, showing hemolytic activity in red blood cells. These proteins are composed of two outer layers of α-helix hairpins wrapped around a β-sheet. With regard to their mode of action, one model proposed that the two outer layers of α-helix hairpins swing away from the β-sheet, allowing insertion of β-strands into the membrane forming a pore after toxin oligomerization. The other model suggested a detergent-like mechanism of action of the toxin on the surface of the lipid bilayer. In this work, we cloned the N- and C-terminal domains form Cyt1Aa and analyzed their effects on Cyt1Aa toxin action. The N-terminal domain shows a dominant negative phenotype inhibiting the in vitro hemolytic activity of Cyt1Aa in red blood cells and the in vivo insecticidal activity of Cyt1Aa against Aedes aegypti larvae. In addition, the N-terminal region is able to induce aggregation of the Cyt1Aa toxin in solution. Finally, the C-terminal domain composed mainly of β-strands is able to bind to the SUV liposomes, suggesting that this region of the toxin is involved in membrane interaction. Overall, our data indicate that the two isolated domains of Cyt1Aa have different roles in toxin action. The N-terminal region is involved in toxin aggregation, while the C-terminal domain is involved in the interaction of the toxin with the lipid membrane.

Cyt1Aa protein from Bacillus thuringiensis (Berliner) serovar israelensis is active against the Mediterranean fruit fly, Ceratitis capitata (Wiedemann)

BACKGROUND

Ceratitis capitata (Wiedemann) is one of the world's most destructive fruit pests. The aim of this study was to ascertain insecticidal activity of Bacillus thuringiensis (Berliner) delta-endotoxins to C. capitata.

RESULTS

Among 42 selected Bacillus strains, only B. thuringiensis serovar israelensis (Bti) solubilised protoxins showed biological activity against C. capitata neonate larvae, whereas Bti spore and crystal mixture was inactive. Insecticidal activity of Bti protoxins was significantly enhanced by incubation with Culex pipiens L. gut extracts. Overdigestion of Bti protoxins with Sesamia nonagrioides (Lefebvre) gut extracts suppressed biological activity against C. capitata, and this correlated with degradation of Cyt toxins. Cyt1Aa solubilised protoxin showed the highest toxicity, LC(50) after 7 days of 4.93 microg cm(-2), while proteolytical processing of Cyt1Aa protoxins by larval gut extracts did not enhance insecticidal activity.

CONCLUSION

The present study provides evidence for the first time of the insecticidal activity of a B. thuringiensis strain against C. capitata and identifies a single delta-endotoxin with potential for controlling this pest.

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

We compared two insecticidal and eight noninsecticidal soil isolates of Bacillus thuringiensis with regard to the solubility of their proteinaceous crystals at alkaline pH values. The protein disulfide contents of the insecticidal and noninsecticidal crystals were equivalent. However, six of the noninsecticidal crystals were soluble only at pH values of >/=12. This lack of solubility contributed to their lack of toxicity. One crystal type which was soluble only at pH >/=12 (strain SHP 1-12) did exhibit significant toxicity to tobacco hornworm larvae when the crystals were presolubilized. In contrast, freshly prepared crystals from the highly insecticidal strain HD-1 were solubilized at pH 9.5 to 10.5, but when these crystals were denatured, by either 8 M urea or autoclave temperatures, they became nontoxic and were soluble only at pH values of >/=12. These changes in toxicity and solubility occurred even though the denatured HD-1 crystals were morphologically indistinguishable from native crystals. Our data are consistent with the view that insecticidal crystals contain distorted, destabilized disulfide bonds which allow them to be solubilized at pH values (9.5 to 10.5) characteristic of lepidopteran and dipteran larval midguts.

Evolution of resistance to the Bacillus sphaericus Bin toxin is phenotypically masked by combination with the mosquitocidal proteins of Bacillus thuringiensis subspecies israelensis

Two insecticidal bacteria are used as larvicides to control larvae of nuisance and vector mosquitoes in many countries, Bacillus thuringiensis ssp. israelensis and B. sphaericus. Field studies show both are effective, but serious resistance, as high as 50 000-fold, has evolved where B. sphaericus is used against Culex mosquitoes. To improve efficacy and deal with even greater potential problems of resistance, we previously developed several recombinant larvicidal bacteria that combine the best mosquitocidal proteins of these bacteria. In the present study, we report laboratory selection studies using our best recombinant strain against larvae of Culex quinquefasciatus. This recombinant, Bti/BsBin, is a strain of B. thuringiensis ssp. israelensis engineered to produce a large amount of the B. sphaericus binary (Bin) toxin, which makes it more than 10-fold as mosquitocidal as the its parental strains. Here we show that larvae exposed to Bti/BsBin failed to develop significant resistance after 30 successive generations of heavy selection pressure. The highest level of resistance obtained at the LC(95) level was 5.2-fold, but declined to less than two-fold at the 35th generation. Testing the selected populations against B. sphaericus alone showed resistance to Bin evolved, but was masked by combination with B. thuringiensis ssp. israelensis. These results suggest that recombinant bacterial strains have improved mosquito and vector management properties compared with the wild-type strains used in current commercial formulations, and should prove useful in controlling important human diseases such as malaria and filariasis on a long-term basis, even when used intensively under field conditions.

Variations in the mosquito larvicidal activities of toxins from Bacillus thuringiensis ssp. israelensis

Comparing activities of purified toxins from Bacillus thuringiensis ssp. israelensis against larvae of seven mosquito species (vectors of tropical diseases) that belong to three genera, gleaned from the literature, disclosed highly significant variations in the levels of LC(50) as well as in the hierarchy of susceptibilities. Similar toxicity comparisons were performed between nine transgenic Gram-negative species, four of which are cyanobacterial, expressing various combinations of cry genes, cyt1Aa and p20, against larvae of four mosquito species as potential agents for biological control. Reasons for inconsistencies are listed and discussed. Standard conditions for toxin isolation and presentation to larvae are sought. A set of lyophilized powders prepared identically from six Escherichia coli clones expressing combinations of four genes displayed toxicities against larvae of three mosquito species, with levels that differed between them but with identical hierarchy.

Evidence of DNA rearrangements in the 128-kilobase pBtoxis plasmid of Bacillus thuringiensis israelensis

BUPM97 is a novel Tunisian isolate of Bacillus thuringiensis israelensis presenting insecticidal activity against Culex pipiens larvae. The delta-endotoxins pattern of this strain was different from that of the reference strain B. thuringiensis israelensis H14. Therefore, the study of its cry genes content was carried out by restriction-fragment-length-polymorphism (RFLP) using specific cry genes probes and by DNA sequencing. It was clearly demonstrated that in the strain BUPM97 the cry4A and cry10A genes were deleted from the B. thuringiensis israelensis 128-kb pBtoxis plasmid. In addition, a strong DNA sequence polymorphism was evidenced in the same plasmid downstream from the cry4B gene. This very particular DNA dynamic evidenced in this new strain of B. thuringiensis israelensis should be taken into consideration, regarding the strain stability during the industrial production of B. thuringiensis bioinsecticides.

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.

Iteron-binding ORF157 and FtsZ-like ORF156 proteins encoded by pBtoxis play a role in its replication in Bacillus thuringiensis subsp. israelensis

We recently identified a minireplicon of pBtoxis from Bacillus thuringiensis subsp. israelensis that contained an operon encoding two novel proteins (ORF156 and ORF157), both of which are required for replication. ORF157 contains a helix-turn-helix motif and shares no homology with known plasmid replication proteins (Rep), and ORF156 contains the signature motif present in FtsZ/tubulin proteins, the latter of which are known to function in cell division and chromosome segregation. Here we show that the minimal sequence composed of four 12-bp imperfect direct repeats (iterons) in the pBtoxis minireplicon was sufficient to replicate a reporter plasmid in B. thuringiensis subsp. israelensis when ORF156 and ORF157 functions were provided in trans. To further investigate the roles of ORF156 and ORF157 in pBtoxis replication, six-histidine-tagged recombinant rORF156 and rORF157 proteins were purified from Escherichia coli and used in electrophoretic mobility shift assays. Our results demonstrated that rORF157, but not rORF156, binds specifically to the pBtoxis iterons, suggesting that ORF157 functions as a Rep protein. Although rORF156 did not bind to the iteron sequence, we showed that it bound to rORF157-DNA complexes. In addition, we showed that rORF156 has GTPase activity characteristic of the FtsZ/tubulin superfamily of proteins. Taken together, these results suggest that the iterons compose the minimal replication origin (ori) of pBtoxis and that ORF157 and ORF156 are involved in the initiation of pBtoxis replication and possibly in the segregation and partitioning of this plasmid to daughter cells.

Mtx toxins synergize Bacillus sphaericus and Cry11Aa against susceptible and insecticide-resistant Culex quinquefasciatus larvae

Two mosquitocidal toxins (Mtx) of Bacillus sphaericus, which are produced during vegetative growth, were investigated for their potential to increase toxicity and reduce the expression of insecticide resistance through their interactions with other mosquitocidal proteins. Mtx-1 and Mtx-2 were fused with glutathione S-transferase and produced in Escherichia coli, after which lyophilized powders of these fusions were assayed against Culex quinquefasciatus larvae. Both Mtx proteins showed a high level of activity against susceptible C. quinquefasciatus mosquitoes, with 50% lethal concentrations (LC(50)) of Mtx-1 and Mtx-2 of 0.246 and 4.13 microg/ml, respectively. The LC(50)s were 0.406 to 0.430 microg/ml when Mtx-1 or Mtx-2 was mixed with B. sphaericus, and synergy improved activity and reduced resistance levels. When the proteins were combined with a recombinant Bacillus thuringiensis strain that produces Cry11Aa, the mixtures were highly active against Cry11A-resistant larvae and resistance was also reduced. The mixture of two Mtx toxins and B. sphaericus was 10 times more active against susceptible mosquitoes than B. sphaericus alone, demonstrating the influence of relatively low concentrations of these toxins. These results show that, similar to Cyt toxins from B. thuringiensis subsp. israelensis, Mtx toxins can increase the toxicity of other mosquitocidal proteins and may be useful for both increasing the activity of commercial bacterial larvicides and managing potential resistance to these substances among mosquito populations.

Cross-resistance spectra of Culex quinquefasciatus resistant to mosquitocidal toxins of Bacillus thuringiensis towards recombinant Escherichia coli expressing genes from B. thuringiensis ssp. israelensis

Sixteen Escherichia coli clones were assayed against susceptible and Bacillus thuringiensis-resistant Culex quinquefasciatus larvae. The clones expressed different combinations of four genes from Bacillus thuringiensis ssp. israelensis; three genes encoded mosquitocidal toxins (Cry11Aa, Cry4Aa and Cyt1Aa) and the fourth encoded an accessory protein (P20). The cross-resistance spectra of the mosquitoes were similar to the profiles for recombinant B. thuringiensis strains expressing B. thuringiensis toxin genes, but with varied toxicity levels. The toxicity of the recombinants towards resistant mosquito larvae was improved when p20 and cyt1Aa were expressed in combination with cry4Aa and/or cry11Aa. Recombinant pVE4-ADRC, expressing cry4Aa, cry11Aa, p20 and cyt1Aa, was the most active against the resistant Culex, and resistance levels did not exceed fourfold. These results indicate that B. thuringiensis ssp. israelensis genes expressed in a heterologous host such as E. coli can be effective against susceptible and B. thuringiensis-resistant larvae and suppress resistance.

The 20-kDa protein of Bacillus thuringiensis subsp. israelensis enhances Bacillus sphaericus 2362 bin toxin synthesis

Improving the amount of protein endotoxins synthesized by Bacillus thuringiensis and B. sphaericus per unit of culture medium is important because higher yields typically correlate with higher insecticidal activity per unit weight of spore/toxin mixtures. Higher levels of synthesis can also result in larger crystals that could persist for longer periods in the environment. Improving endotoxin production in B. thuringiensis can be achieved by manipulating genetic elements that regulate protein synthesis at the transcriptional, translational, and even posttranslational levels. In the present study, we used a combination of genetic elements to improve yields of B. sphaericus 2362 binary toxin (Bin) in B. thuringiensis. Our results show that a 20-kDa chaperone-like protein, which occurs as the third open-reading frame in the cry11Aa operon, improves Bin yields when expression of the genes encoding this binary toxin is driven by the native bin promoter, cyt1A promoters, or a novel cyt1A-p/STAB-SD expression system, the latter of which yields maximal levels of Bin synthesis. The 20-kDa helper protein increased Bin toxin levels in B. thuringiensis by as much as 53% and concomitant toxicity by at least 90% when Bin was produced using the cyt1A promoters.