Effect of accessory proteins P19 and P20 on cytolytic activity of Cyt1Aa from Bacillus thuringiensis subsp. israelensis in Escherichia coli

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.

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.

A novel anti-dipteran Bacillus thuringiensis strain: Unusual Cry toxin genes in a highly dynamic plasmid environment

Bacillus thuringiensis emerged as a major bioinsecticide on the global market. It offers a valuable alternative to chemical products classically utilized to control pest insects. Despite the efficiency of several strains and products available on the market, the scientific community is always on the lookout for novel toxins that can replace or supplement the existing products. In this study, H3, a novel B. thuringiensis strain showing mosquitocidal activity, was isolated from Lebanese soil and characterized at an in vivo, genomic and proteomic levels. H3 parasporal crystal is toxic on its own but displays an unusual killing profile with a higher LC50 than the reference B. thuringiensis serovar israelensis crystal proteins. In addition, H3 has a different toxicity order: it is more toxic to Aedes albopictus and Anopheles gambiae than to Culex pipiens Whole genome sequencing and crystal analysis revealed that H3 can produce eleven novel Cry proteins, eight of which are assembled in genes with an orf1-gap-orf2 organization, where orf2 is a potential Cry4-type crystallization domain. Moreover, pH3-180, the toxin-carrying plasmid, holds a wide repertoire of mobile genetic elements that amount to ca 22% of its size., including novel insertion sequences and class II transposable elements Two other large plasmids present in H3 carry genetic determinants for the production of many interesting molecules - such as chitinase, cellulase and bacitracin - that may add up to H3 bioactive properties. This study therefore reports a novel mosquitocidal Bacillus thuringiensis strain with unusual Cry toxin genes in a rich mobile DNA environment.IMPORTANCE Bacillus thuringiensis, a soil entomopathogenic bacteria, is at the base of many sustainable eco-friendly bio-insecticides. Hence stems the need to continually characterize insecticidal toxins. H3 is an anti-dipteran B. thuringiensis strain, isolated from Lebanese soil, whose parasporal crystal contains eleven novel Cry toxins and no Cyt toxins. In addition to its individual activity, H3 showed potential as a co-formulant with classic commercialized B. thuringiensis products, to delay the emergence of resistance and to shorten the time required for killing. On a genomic level, H3 holds three large plasmids, one of which carries the toxin-coding genes, with four occurrences of the distinct orf1-gap-orf2 organization. Moreover, this plasmid is extremely rich in mobile genetic elements, unlike its two co-residents. This highlights the important underlying evolutionary traits between toxin-carrying plasmids and the adaptation of a B. thuringiensis strain to its environment and insect host spectrum.

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.

Role of plasmid plasticity and mobile genetic elements in the entomopathogen Bacillus thuringiensis serovar israelensis

Bacillus thuringiensis is a well-known biopesticide that has been used for more than 80 years. This spore-forming bacterium belongs to the group of Bacillus cereus that also includes, among others, emetic and diarrheic pathotypes of B. cereus, the animal pathogen Bacillus anthracis and the psychrotolerant Bacillus weihenstephanensis. Bacillus thuringiensis is rather unique since it has adapted its lifestyle as an efficient pathogen of specific insect larvae. One of the peculiarities of B. thuringiensis strains is the extent of their extrachromosomal pool, with strains harbouring more than 10 distinct plasmid molecules. Among the numerous serovars of B. thuringiensis, 'israelensis' is certainly emblematic since its host spectrum is apparently restricted to dipteran insects like mosquitoes and black flies, vectors of human and animal diseases such as malaria, yellow fever, or river blindness. In this review, the putative role of the mobile gene pool of B. thuringiensis serovar israelensis in its pathogenicity and dedicated lifestyle is reviewed, with specific emphasis on the nature, diversity, and potential mobility of its constituents. Variations among the few related strains of B. thuringiensis serovar israelensis will also be reported and discussed in the scope of this specialised insect pathogen, whose lifestyle in the environment remains largely unknown.

Expression of a Synthetic Gene for the Major Cytotoxin (Cyt1Aa) of Bacillus thuringiensis subsp. israelensis in the Chloroplast of Wild-Type Chlamydomonas

Chlamydomonas reinhardtii (Chlamydomonas) strains that are toxic to mosquito larvae because they express chloroplast transgenes that are based on the mosquitocidal proteins of Bacillus thuringiensis subsp. israelensis (Bti) could be very useful in mosquito control. Chlamydomonas has several advantages for this approach, including genetic controls not generally available with industrial algae. The Bti toxin is produced by sporulating bacteria and has been used for mosquito control for >30 years without creating highly resistant mosquito populations. The suite of toxins is four main proteins: three Cry proteins and the cytotoxic Cyt1Aa (27 kDa). Cyt1Aa is not very toxic to mosquitoes by itself, but it prevents the development of resistance. The production of Cyt1Aa in other microbes, however, has been challenging due to its affinity for certain membrane phospholipids. Here we report on the production of recombinant Cyt1Aa (rCyt1A) in the chloroplast of photosynthetic Chlamydomonas at levels of at least 0.3% total protein. Live cell bioassays demonstrated toxicity of the rCyt1Aa Chlamydomonas to larvae of Aedes aegypti. We also expressed the chloroplast cyt1Aa gene in a wild-type Chlamydomonas strain (21 gr) that can grow on nitrate. These results have implications for developing a Chlamydomonas strain that will be toxic to mosquito larvae but will not induce strongly resistant populations.

Expression of Bacillus thuringiensis cytolytic toxin (Cyt2Ca1) in citrus roots to control Diaprepes abbreviatus larvae

Diaprepes abbreviatus (L.) is an important pest of citrus in the USA. Currently, no effective management strategies of D. abbreviatus exist in citriculture, and new methods of control are desperately sought. To protect citrus against D. abbreviatus a transgenic citrus rootstock expressing Bacillus thuringiensis Cyt2Ca1, an insect toxin protein, was developed using Agrobacterium-mediated transformation of 'Carrizo' citrange [Citrus sinensis (L) Osbeck Poncirus trifoliate (L) Raf]. The transgenic citrus root stock expressed the cytolytic toxin Cyt2Ca1 constitutively under the control of a 35S promoter in the transgenic Carrizo citrange trifoliate hybrid including the roots that are the food source of larval D. abbreviatus. The engineered citrus was screened by Western blot and RT-qPCR analyses for cyt2Ca1 and positive citrus identified. Citrus trees expressing different levels of cyt2Ca1 transcripts were identified (Groups A-C). High expression of the toxin in the leaves (10⁹ transcripts/ng RNA), however, retarded plant growth. The transgenic plants were grown in pots and the roots exposed to 3week old D. abbreviatus larvae using no-choice plant bioassays. Three cyt2Ca1 transgenic plants were identified that sustained less root damage belonging to Group B and C. One plant caused death to 43% of the larvae that fed on its roots expressed 8×10⁶cyt2Ca1 transcripts/ng RNA. These results show, for the first time, that Cyt2Ca1 expressed in moderate amounts by the roots of citrus does not retard citrus growth and can protect it from larval D. abbreviatus.

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.

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.

New Cyt-like δ-endotoxins from Dickeya dadantii: structure and aphicidal activity

In the track of new biopesticides, four genes namely cytA, cytB, cytC and cytD encoding proteins homologous to Bacillus thuringiensis (Bt) Cyt toxins have been identified in the plant pathogenic bacteria Dickeya dadantii genome. Here we show that three Cyt-like δ-endotoxins from D. dadantii (CytA, CytB and CytC) are toxic to the pathogen of the pea aphid Acyrthosiphon pisum in terms of both mortality and growth rate. The phylogenetic analysis of the comprehensive set of Cyt toxins available in genomic databases shows that the whole family is of limited taxonomic occurrence, though in quite diverse microbial taxa. From a structure-function perspective the 3D structure of CytC and its backbone dynamics in solution have been determined by NMR. CytC adopts a cytolysin fold, structurally classified as a Cyt2-like protein. Moreover, the identification of a putative lipid binding pocket in CytC structure, which has been probably maintained in most members of the Cyt-toxin family, could support the importance of this lipid binding cavity for the mechanism of action of the whole family. This integrative approach provided significant insights into the evolutionary and functional history of D. dadantii Cyt toxins, which appears to be interesting leads for biopesticides.

Draft genome sequences of two Bacillus thuringiensis strains and characterization of a putative 41.9-kDa insecticidal toxin

In this work, we report the genome sequencing of two Bacillus thuringiensis strains using Illumina next-generation sequencing technology (NGS). Strain Hu4-2, toxic to many lepidopteran pest species and to some mosquitoes, encoded genes for two insecticidal crystal (Cry) proteins, cry1Ia and cry9Ea, and a vegetative insecticidal protein (Vip) gene, vip3Ca2. Strain Leapi01 contained genes coding for seven Cry proteins (cry1Aa, cry1Ca, cry1Da, cry2Ab, cry9Ea and two cry1Ia gene variants) and a vip3 gene (vip3Aa10). A putative novel insecticidal protein gene 1143 bp long was found in both strains, whose sequences exhibited 100% nucleotide identity. The predicted protein showed 57 and 100% pairwise identity to protein sequence 72 from a patented Bt strain (US8318900) and to a putative 41.9-kDa insecticidal toxin from Bacillus cereus, respectively. The 41.9-kDa protein, containing a C-terminal 6× HisTag fusion, was expressed in Escherichia coli and tested for the first time against four lepidopteran species (Mamestra brassicae, Ostrinia nubilalis, Spodoptera frugiperda and S. littoralis) and the green-peach aphid Myzus persicae at doses as high as 4.8 µg/cm² and 1.5 mg/mL, respectively. At these protein concentrations, the recombinant 41.9-kDa protein caused no mortality or symptoms of impaired growth against any of the insects tested, suggesting that these species are outside the protein’s target range or that the protein may not, in fact, be toxic. While the use of the polymerase chain reaction has allowed a significant increase in the number of Bt insecticidal genes characterized to date, novel NGS technologies promise a much faster, cheaper and efficient screening of Bt pesticidal proteins.

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.

Draft Genome Sequence of Bacillus thuringiensis Serovar Tolworthi Strain Na205-3, an Isolate Toxic for Helicoverpa armigera

We report here the complete annotated 6,510,053-bp draft genome sequence of Bacillus thuringiensis serovar tolworthi strain Na205-3, which is toxic for Helicoverpa armigera. This strain potentially contains nine insecticidal toxin genes homologous to cry1Aa12, cry1Ab1, cry1Ab8, cry1Ba1, cry1Af1, cry1Ia10, vip1Bb1, vip2Ba2, and vip3Aa6.

Tandem repeats in a new toxin gene from Bacillus thuringiensis and in other cry11-like genes

A new gene, cry11Bb2 from a field isolate of Bacillus thuringiensis, was cloned for expression in Escherichia coli. The encoded protein, with a deduced molecular mass of 89.5 kDa, exhibits 97 and 79% identities with the overlap regions of Cry11Bb1 from B. thuringiensis ssp. medellin and Cry11Ba1 from ssp. jegathesan, respectively. It is however longer than Cry11Bb1 by 42 amino acids in its carboxy-terminus, of which 32 comprise 2 tandem repeats additional to the 5 existing in the latter polypeptide. Possible roles for this recurrent motif among Cry toxins and their accessory proteins, and for their encoding genes are proposed.

Expression in Escherichia coli of the native cyt1Aa from Bacillus thuringiensis subsp. israelensis

The gene cyt1Aa is one of the genes in the complex determining the mosquito larvicidity of Bacillus thuringiensis subsp. israelensis. Previous cloning in Escherichia coli resulted in a 48-bp addition upstream, encoding a chimera. Here, cyt1Aa was recloned without the artifact, and its toxicity against Aedes aegypti larvae and host E. coli cells was retested.

A 1.1-kilobase region downstream of the bin operon in Bacillus sphaericus strain 2362 decreases bin yield and crystal size in strain 2297

The 2297 strain of Bacillus sphaericus produces a crystal of the Bin (binary) toxin that is approximately fourfold larger than that of strain 2362, the strain currently used in VectoLex, a commercial mosquito larvicide. Comparison of the regions downstream from the bin operon in these two strains showed that strain 2362 contained a 1.6-kb region with four orf genes not found in strain 2297. Insertion of a 1.1-kb portion of this region from strain 2362 by homologous recombination downstream from the bin operon in strain 2297 reduced Bin toxin production by 50 to 70% and toxicity to fourth-instar larvae of Culex quinquefasciatus by 68%. These results suggest that the 1.6-kb region downstream from the bin operon in B. sphaericus 2362 is responsible for the lower Bin yield and smaller crystal size characteristic of this strain.

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.

Larvicidal activities against agricultural pests of transgenic Escherichia coli expressing combinations of four genes from Bacillus thuringiensis

The genes cry1Ac and cry1Ca from Bacillus thuringiensis subsps. kurstaki HD-73 and aizawai 4J4, respectively, encoding delta-endotoxins against lepidopteran larvae were isolated, cloned and expressed in Escherichia coli, with and without cyt1Aa (encoding cytolytic protein) and p20 (accessory protein) from subsp. israelensis. Nine combinations of the genes under control of an early T7, P A1 inducible promoter, produced the encoding proteins. Toxicities were examined against larvae of three major agricultural pests: Pectinophora gossypiella, Helicoverpa armigera and Spodoptera littoralis. The clones expressing cyt1Aa, with or without p20, were not toxic. The clone expressing cry1Ac (pBt-1A) was the most toxic to P. gossypiella (LC50 of 0.27 x 10(8) cells g(-1)). Clone pBt-1CA expressing cry1Ca and cry1Ac displayed the highest toxicity (LC50 of 0.12 x 10(8) cells ml(-1)) against S. littoralis. Clone pBt-1CARCy expressing all four genes (cry1Ca, cry1Ac, p20, cyt1Aa) in tandem exhibited the highest toxicity to H. armigera (LC50 of 0.16 x 10(8) cells ml(-1)). Cyt1Aa failed to raise the toxicity of these Cry toxins against P. gossypiella and S. littoralis but significantly enhanced toxicity against H. armigera. Two additional clones expressing either cry1Ac or cry1Ca under tandem promoters, P A1 and P psbA (constitutive), displayed significantly higher toxicities (7.5- to 140-fold) than their counterparts with P A1 alone, reducing the LC50 values to below 10(7) cells ml(-1).

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.

Cyt1Ca from Bacillus thuringiensis subsp. israelensis: production in Escherichia coli and comparison of its biological activities with those of other Cyt-like proteins

The larvicidal activity ofBacillus thuringiensissubsp.israelensisagainst dipteran larvae is determined by four major polypeptides of the parasporal crystalline body produced during sporulation. Cyt1Aa shows the lowest toxicity when used alone but is the most synergistic with any of the other proteins. The sequence of the plasmid pBtoxis, which contains all the toxin genes in this subspecies, revealed a newcyt-like coding sequence namedcyt1Ca. In addition to the Cyt-like region, the predicted Cyt1Ca contained an extra domain at the C terminus, which appeared to be aβ-trefoil carbohydrate-binding motif, as found in several ricin-like toxins. The gene was PCR-amplified from pBtoxis and cloned in several vectors, allowing high-level expression inEscherichia coli. Cyt1Ca was purified by nickel-nitrilotriacetic acid affinity chromatography, characterized, and its biological activity was determined. Toxicity against larvae ofAedes aegyptiof Cyt1Ca in recombinantE. colicells was compared with that of Cyt1Aa and Cyt2Ba, and the ability of these proteins to enhance the activity of Cry4Aa was assessed. Although Cyt2Ba appeared able to interact with Cry4Aa, no activity for Cyt1Ca was observed, even when produced in truncated form. Furthermore, in contrast to Cyt1Aa, Cyt1Ca did not lyse sheep erythrocytes, and it was not bactericidal to the host cell.

Transfer and expression of the mosquitocidal plasmid pBtoxis in Bacillus cereus group strains

The toxicity of Bacillus thuringiensis subsp. israelensis to dipteran larvae (mosquitoes and black flies) depends on the presence of the pBtoxis plasmid. In this paper, two antibiotic resistance tagged pBtoxis were transferred by conjugation to other Bacillus cereus group strains. Among 15 potential recipients, only a lepidopteran active B. thuringiensis subspecies kurstaki and a B. cereus strain received the plasmid pBtoxis with a low transfer rate of about 10(-8) transconjugants/recipient. The resulting B. thuringiensis subspecies kurstaki transconjugant was active to both lepidopteran and dipteran targets and the B. cereus transconjugant was active against dipteran insects. Phase contrast microscopy showed that the B. cereus transconjugants could produce only round crystalline inclusion bodies while B. thuringiensis subspecies kurstaki transconjugant could produce both round and bipyramidal crystals during sporulation. SDS-PAGE revealed that all the major mosquitocidal proteins from pBtoxis could express in the two transconjugants, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa and Cyt1Aa. However, none of the experiment showed any indications of mobilising abilities of pBtoxis. The limited number of strains, which could receive and maintain pBtoxis using a conjugational helper plasmid, indicates a very narrow host range of the B. thuringiensis subsp. israelensis pBtoxis plasmid.

Partial restoration of antibacterial activity of the protein encoded by a cryptic open reading frame (cyt1Ca) from Bacillus thuringiensis subsp. israelensis by site-directed mutagenesis

Insecticidal crystal proteins of Bacillus thuringiensis belong to two unrelated toxin families: receptor-specific Cry toxins against insects and Cyt toxins that lyse a broad range of cells, including bacteria, via direct binding to phospholipids. A new cyt-like open reading frame (cyt1Ca) encoding a 60-kDa protein, has recently been discovered (C. Berry et al., Appl. Environ. Microbiol. 68:5082-5095, 2002). Cyt1Ca displays the structure of a two-domain fusion protein: the N-terminal moiety resembles the full-length Cyt toxins, and the C-terminal moiety is similar to the receptor-binding domains of several ricin-like toxins, such as Mtx1. Neither the larvicidal activity of cyt1Ca expressed in Escherichia coli nor the hemolytic effect of His-tagged purified Cyt1Ca has been observed (R. Manasherob et al., unpublished). This was attributed to five amino acid differences between the sequences of its N-terminal moiety and Cyt1Aa. The 3' end of cyt1Ca was truncated (removing the ricin-binding domain of Cyt1Ca), and six single bases were appropriately changed by site-directed mutagenesis, sequentially replacing the non-charged amino acids by charged ones, according to Cyt1Aa, to form several versions. Expression of these mutated cyt1Ca versions caused loss of the colony-forming ability of the corresponding E. coli cells to different extents compared with the original gene. In some mutants this antibacterial effect was associated by significant distortion of cell morphology and in others by generation of multiple inclusion bodies spread along the cell envelope. The described deleterious effects of mutated cyt1Ca versions against E. coli may reflect an evolutionary relationship between Cyt1Aa and Cyt1Ca.

Mosquito larvicidal activity of transgenic Anabaena PCC 7120 expressing toxin genes from Bacillus thuringiensis subsp. israelensis

Genes encoding the mosquito larvicidal toxins Cry4Aa, Cry11Aa, Cyt1Aa and the regulatory P20 from Bacillus thuringiensis subsp. israelensis were introduced into the nitrogen-fixing, filamentous cyanobacterium Anabaena PCC 7120 for expression under control of two strong promoters P(psbA) and P(A1). The clone pRVE4-ADRC displayed toxicity against fourth-instar larvae of Aedes aegypti, the highest ever achieved in cyanobacteria. It was about 2.5-fold more toxic than the respective clone without cyt1Aa [Wu et al., Appl. Environ. Microbiol. 63 (1997) 4971-4975]. Cyt1Aa synergized the combination of Crys by about five-fold. Consistently, the lethal times exerted by pRVE4-ADRC were also reduced (it killed exposed larvae more quickly). This clone may become a useful biological control agent which reduces the probability of resistance development in the target organisms [Wirth et al., Proc. Natl. Acad. Sci. USA 94 (1997) 10536-10540].

Compaction of the Escherichia coli nucleoid caused by Cyt1Aa

Compaction of the Escherichia coli nucleoid in the cell's centre was associated with the loss of colony-forming ability; these effects were caused by induction of Cyt1Aa, the cytotoxic 27 kDa protein from Bacillus thuringiensis subsp. israelensis. Cyt1Aa-affected compaction of the nucleoids was delayed but eventually more intense than compaction caused by chloramphenicol. The possibility that small, compact nucleoids in Cyt1Aa-expressing cells resulted in DNA replication run-out and segregation following cell division was ruled out by measuring relative nucleoid length. Treatments with membrane-perforating substances other than Cyt1Aa did not cause such compaction of the nucleoids, but rather the nucleoids overexpanded to occupy nearly all of the cell volume. These findings support the suggestion that, in addition to its perforating ability, Cyt1Aa causes specific disruption of nucleoid associations with the cytoplasmic membrane. In situ immunofluorescence labelling with Alexa did not demonstrate a great amount of Cyt1Aa associated with the membrane. Clear separation between Alexa-labelled Cyt1Aa and 4',6-diamidino-2-phenylindole (DAPI)-stained DNA indicates that the nucleoid does not bind Cyt1Aa. Around 2 h after induction, nucleoids in Cyt1Aa-expressing cells started to decompact and expanded to fill the whole cell volume, most likely due to partial cell lysis without massive peptidoglycan destruction.

Activity of free and clay-bound insecticidal proteins from Bacillus thuringiensis subsp. israelensis against the mosquito Culex pipiens

Bacillus thuringiensis subsp. israelensis produces parasporal insecticidal crystal proteins (ICPs) that have larvicidal activity against some members of the order Diptera, such as blackflies and mosquitoes. Hydrolysis of the ICPs in the larval gut results in four major proteins with a molecular mass of 27, 65, 128, and 135 kDa. Toxicity is caused by synergistic interaction between the 25-kDa protein (proteolytic product of the 27-kDa protein) and one or more of the higher-molecular-mass proteins. Equilibrium adsorption of the proteins on the clay minerals montmorillonite and kaolinite, which are homoionic to various cations, was rapid (<30 min for maximal adsorption), increased with protein concentration and then reached a plateau (68 to 96% of the proteins was adsorbed), was significantly lower on kaolinite than on montmorillonite, and was not significantly affected by the valence of the cation to which the clays were homoionic. Binding of the toxins decreased as the pH was increased from 6 to 11, and there was 35 to 66% more binding in phosphate buffer at pH 6 than in distilled water at pH 6 or 7.2. Only 2 to 12% of the adsorbed proteins was desorbed by two washes with water; additional washings desorbed no more toxins, indicating that they were tightly bound. Formation of clay-toxin complexes did not alter the structure of the proteins, as indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the equilibrium supernatants and desorption washes and by dot blot enzyme-linked immunosorbent assay of the complexes, which was confirmed by enhanced chemiluminescence Western blot analysis. Free and clay-bound toxins resulted in 85 to 100% mortality of the mosquito Culex pipiens. Persistence of the bound toxins in nonsterile water after 45 days was significantly greater (mortality of 63% +/- 12.7%) than that of the free toxins (mortality of 25% +/- 12.5%).

Complete sequence and organization of pBtoxis, the toxin-coding plasmid of Bacillus thuringiensis subsp. israelensis

The entire 127,923-bp sequence of the toxin-encoding plasmid pBtoxis from Bacillus thuringiensis subsp. israelensis is presented and analyzed. In addition to the four known Cry and two known Cyt toxins, a third Cyt-type sequence was found with an additional C-terminal domain previously unseen in such proteins. Many plasmid-encoded genes could be involved in several functions other than toxin production. The most striking of these are several genes potentially affecting host sporulation and germination and a set of genes for the production and export of a peptide antibiotic.

Toxicity and synergism in transgenic Escherichia coli expressing four genes from Bacillus thuringiensis subsp. israelensis

The genes cyt1Aa and p20, encoding, respectively, cytolytic and accessory proteins of Bacillus thuringiensis subsp. israelensis, were introduced into previously constructed clones expressing cry4Aa and cry11Aa in Escherichia coli (Ben-Dov et al., 1995). Fifteen clones with all possible combinations of the four genes were obtained and found to express the genes included. Two new combinations, pVE4-ADRC and pVE4-ARC, expressing cyt1Aa, p20 and cry4Aa, with or without cry11Aa, respectively, were more toxic than their counterparts without cyt1Aa. They displayed the highest toxicity against Aedes aegypti larvae ever reached in transgenic bacteria. Five out of the six clones (except pVE4-DC) containing cry4Aa or cry11Aa (with or without p20) displayed varying levels of synergism with cyt1Aa: they are 1.5-to 34-fold more toxic than the respective clones without cyt1Aa against exposed larvae. Their lethal times also decreased (they kill larvae quicker), more so at higher cell concentrations. These clones are anticipated to dramatically reduce the likelihood of resistant development in the target organisms (Wirth et al., 1997).