Molecular cloning and characterization of the insecticidal crystal protein gene of Bacillus thuringiensis var. tenebrionis

Elucidating the genomic history of commercially used Bacillus thuringiensis subsp. tenebrionis strain NB176

Bacillus thuringiensis subsp. tenebrionis (Btt) produces a coleopteran-specific crystal protoxin protein (Cry3Aa δ-endotoxin). After its discovery in 1982, the strain NB125 (DSM 5526) was eventually registered in 1990 to control the Colorado potato beetle (Leptinotarsa decemlineata). Gamma-irradiation of NB125 resulted in strain NB176-1 (DSM 5480) that exhibited higher cry3Aa production and became the active ingredient of the plant protection product Novodor^® FC. Here, we report a comparative genome analysis of the parental strain NB125, its derivative NB176-1 and the current commercial production strain NB176. The entire genome sequences of the parental and derivative strains were deciphered by a hybrid de novo approach using short (Illumina) and long (Nanopore) read sequencing techniques. Genome assembly revealed a chromosome of 5.4 to 5.6 Mbp and six plasmids with a size range from 14.9 to 250.5 kbp for each strain. The major differences among the original NB125 and the derivative strains NB176-1 and NB176 were an additional copy of the cry3Aa gene, which translocated to another plasmid as well as a chromosomal deletion (~ 178 kbp) in NB176. The assembled genome sequences were further analyzed in silico for the presence of virulence and antimicrobial resistance (AMR) genes.

The distinct properties of natural and GM cry insecticidal proteins

The Cry toxins are a family of crystal-forming proteins produced by the bacterium Bacillus thuringiensis. Their mode of action is thought to be to create pores that disrupt the gut epithelial membranes of juvenile insects. These pores allow pathogen entry into the hemocoel, thereby killing the insect. Genes encoding a spectrum of Cry toxins, including Cry mutants, Cry chimaeras and other Cry derivatives, are used commercially to enhance insect resistance in genetically modified (GM) crops. In most countries of the world, such GM crops are regulated and must be assessed for human and environmental safety. However, such risk assessments often do not test the GM crop or its tissues directly. Instead, assessments rely primarily on historical information from naturally occurring Cry proteins and on data collected on Cry proteins (called 'surrogates') purified from laboratory strains of bacteria engineered to express Cry protein. However, neither surrogates nor naturally occurring Cry proteins are identical to the proteins to which humans or other nontarget organisms are exposed by the production and consumption of GM plants. To-date there has been no systematic survey of these differences. This review fills this knowledge gap with respect to the most commonly grown GM Cry-containing crops approved for international use. Having described the specific differences between natural, surrogate and GM Cry proteins this review assesses these differences for their potential to undermine the reliability of risk assessments. Lastly, we make specific recommendations for improving risk assessments.

The red flour beetle as a model for bacterial oral infections

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

Expression of cry3A gene and its toxicity against Asian Gray Weevil Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae)

Coleopterans are the most damaging pests of many agricultural and forestry crops; there is an urgent need to develop effective biopesticides against these insects. Enhancers of Bt toxicity typify an opportunity to improve currently available commercial products into more effective control agents against diverse pests. A 1.9 kb DNA fragment, PCR amplified from native isolates of Bt using cry3A gene specific primers was cloned in expression vector pQE-80L and then used for transformation of Escherichia coli M15 cells. The sequence of the cloned crystal protein gene showed almost complete homology with a Coleopteran active Cry3A toxin gene with 117 mutations scattered in different domain regions encoding a protein of 645 amino acid residues in length, with a predicted molecular mass of 77.4 kDa. Phylogenetic analysis could be compulsive for new/novel Bacillus thuringiensis strains, allowing them to be grouped with related Cry proteins. The toxicity of Bt protein was determined against Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae) LC50 152 ng cm(-2). Genes coding for Coleopteran active Cry3A proteins have been isolated and their efficient expression will provide the tools necessary to increase the efficacy of Cry-based biopesticide against economically important beetles.

Transgenic approaches to western corn rootworm control

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a significant corn pest throughout the United States corn belt. Rootworm larvae feed on corn roots causing yield losses and control expenditures that are estimated to exceed US$1 billion annually. Traditional management practices to control rootworms such as chemical insecticides or crop rotation have suffered reduced effectiveness due to the development of physiological and behavioral resistance. Transgenic maize expressing insecticidal proteins are very successful in protecting against rootworm damage and preserving corn yield potential. However, the high rate of grower adoption and early reliance on hybrids expressing a single mode of action and low-dose traits threatens the durability of commercialized transgenic rootworm technology for rootworm control. A summary of current transgenic approaches for rootworm control and the corresponding insect resistance management practices is included. An overview of potential new modes of action based on insecticidal proteins, and especially RNAi targeting mRNA coding for essential insect proteins is provided.

Enhancement of Bacillus thuringiensis Cry3Aa and Cry3Bb toxicities to coleopteran larvae by a toxin-binding fragment of an insect cadherin

The Cry3Aa and Cry3Bb insecticidal proteins of Bacillus thuringiensis are used in biopesticides and transgenic crops to control larvae of leaf-feeding beetles and rootworms. Cadherins localized in the midgut epithelium are identified as receptors for Cry toxins in lepidopteran and dipteran larvae. Previously, we discovered that a peptide of a toxin-binding cadherin expressed in Escherichia coli functions as a synergist for Cry1A toxicity against lepidopteran larvae and Cry4 toxicity against dipteran larvae. Here we report that the fragment containing the three most C-terminal cadherin repeats (CR) from the cadherin of the western corn rootworm binds toxin and enhances Cry3 toxicity to larvae of naturally susceptible species. The cadherin fragment (CR8 to CR10 [CR8-10]) of western corn rootworm Diabrotica virgifera virgifera was expressed in E. coli as an inclusion body. By an enzyme-linked immunosorbent microplate assay, we demonstrated that the CR8-10 peptide binds alpha-chymotrypsin-treated Cry3Aa and Cry3Bb toxins at high affinity (11.8 nM and 1.4 nM, respectively). Coleopteran larvae ingesting CR8-10 inclusions had increased susceptibility to Cry3Aa or Cry3Bb toxin. The Cry3 toxin-enhancing effect of CR8-10 was demonstrated for Colorado potato beetle Leptinotarsa decemlineata, southern corn rootworm Diabrotica undecimpunctata howardi, and western corn rootworm. The extent of Cry3 toxin enhancement, which ranged from 3- to 13-fold, may have practical applications for insect control. Cry3-containing biopesticides that include a cadherin fragment could be more efficacious. And Bt corn (i.e., corn treated with B. thuringiensis to make it resistant to pests) coexpressing Cry3Bb and CR8-10 could increase the functional dose level of the insect toxic activity, reducing the overall resistance risk.

An engineered chymotrypsin/cathepsin G site in domain I renders Bacillus thuringiensis Cry3A active against Western corn rootworm larvae

The western corn rootworm remains one of the most important pests of corn in the United States despite the use of many pest management tools. Cry3A, the first coleopteran-active Bacillus thuringiensis toxin isolated, has not been useful for control of the corn rootworm pest complex. Modification of Cry3A so that it contained a chymotrypsin/cathepsin G protease recognition site in the loop between alpha-helix 3 and alpha-helix 4 of domain I, however, resulted in consistent activity of the toxin ("mCry3A") against neonate western corn rootworm. In vitro chymotrypsin digests showed that there was a substantial difference between the enzyme sensitivity of mCry3A and the enzyme sensitivity of Cry3A, with mCry3A rapidly converted from a 67-kDa form to a approximately 55-kDa form. The introduced protease site was also recognized in vivo, where the approximately 55-kDa form of mCry3A toxin was rapidly generated and associated with the membrane fraction. After a point mutation in mcry3A that resulted in the elimination of the native domain I chymotrypsin site (C terminal to the introduced chymotrypsin/cathepsin G protease site of mCry3A), the in vitro and in vivo digestion patterns remained the same, demonstrating that the introduced site was required for the enhanced activity. Also, 55-kDa mCry3A generated by cleavage with chymotrypsin exhibited specific binding to western corn rootworm brush border membrane, whereas untreated 67-kDa mCry3A did not. These data indicate that the mCry3A toxicity for corn rootworm larvae was due to the introduction of a chymotrypsin/cathepsin G site, which enhanced cleavage and subsequent binding of the activated toxin to midgut cells.

Inorganic phosphate has a crucial effect on Cry3Aa delta-endotoxin production

AIMS

The study aimed at increasing Cry3Aa delta-endotoxin production by a local isolate of Bacillus thuringiensis (B.t. strain Mm2). To this end, different nutritional conditions were tested for their effects on Cry3Aa yields.

METHODS AND RESULTS

Bacillus thuringiensis Mm2 was grown by shaking at 30 degrees C in different media. Samples were taken from the cultures at intervals and used for protein extraction. SDS-PAGE was performed for toxin analysis. Inclusion of inorganic phosphate (Pi) into the Difco's sporulation medium at an increased level of 200 mmol l-1 caused a fivefold increase (from 3 to 15.6 microg ml-1) in toxin production. Omission of FeSO4 from the medium decreased this yield by half. Resuspension experiments suggested catabolite repression of toxin biosynthesis by glucose. The inclusion of high Pi invariably increased toxin synthesis, even in the absence of sugars.

CONCLUSIONS

Inorganic phosphate had the most striking effect on toxin biosynthesis. Iron effect was found to be unique to our isolate whereas Pi effect seemed to be common to the biosynthesis of Cry3Aa-type toxins. Stimulation of toxin synthesis by Pi did not seem to represent a relief from glucose repression.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacillus thuringiensis is the most versatile biopesticide for use in pest management. Regarding cost-effectiveness of related fermentations, high Pi supplement drastically increases Coleoptera-specific toxin synthesis.

Efficient expression of vip184DeltaP gene under the control of promoters plus Shine-Dalgarno (SD) sequences of cry genes from Bacillus thuringiensis

AIMS

To compare vip184DeltaP gene expression time course and Vip184 protein yield under the control of promoters and Shine-Dalgarno (SD) sequences of vip184, cry3A and cry1A gene from Bacillus thuringiensis respectively.

METHODS AND RESULTS

Derived from the shuttle vector pHT3101, recombinant plasmids pHPT3, pHTP3A(Delta)P and pHTP1A(Delta)P were constructed with the native vip184 gene and the vip184(Delta)P gene, either under the control of promoters and SD sequences of cry3A or cry1A genes. When the above plasmids were transformed into an acrystalliferous B. thuringiensis strain Cry(-)B, their expression time course were consistent with those of vip184, cry3A and cry1A gene respectively. The maximum yields of Vip184 protein were increased when under the control of promoters plus SD sequences of cry3A and cry1A gene.

CONCLUSIONS

The results showed that both cry3A and cry1A promoter/SD sequence combinations were able to enhance synthesis of Vip184 and change its expression time course.

SIGNIFICANCE AND IMPACT OF THE STUDY

Both cry3A and cry1A promoter/SD systems offer a method for improving the expression efficacy of the vip184 gene in B. thuringiensis and it is possible to co-express the vip184 gene and cry genes and accumulate Vip184 in the form of inclusion bodies by these systems in order to construct novel useful B. thuringiensis engineered strains.

Cloning and expression of the vegetative insecticidal protein (vip3V) gene of Bacillus thuringiensis in Escherichia coli

A genomic library of Bacillus thuringiensis var. kurstaki (B.t.k.) was constructed and a positive clone harboring the full-length gene encoding a novel vegetative insecticidal protein (Vip3V) was characterized. The vip3V gene was subcloned into pET-22b(+) vector and overexpressed in Escherichia coli to an extent of about 30% of the total protein. While transcription was influenced by the T7 promoter of the vector, synthesis of Vip3V in E. coli host occurred from the B.t.k. ribosomal binding site (rbs) found 917bp downstream of the insert and not from the E. coli rbs of the vector. The expressed Vip3V protein was found in the soluble and periplasmic fractions as well as in the inclusion bodies. A simplified anion-exchange chromatographic method for the purification of Vip3V using step gradient or one-step elution was developed. The purified protein showed broad-spectrum activity against some of the lepidopteran larvae tested and did not show any activity against the larvae of silkworm (Bombyx mori) and mosquito (Culex quinquefasciatus).

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

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

Molecular genetic manipulation of truncated Cry1C protein synthesis in Bacillus thuringiensis to improve stability and yield

Cry1 protoxins of Bacillus thuringiensis are insecticidal 135-kDa proteins synthesized and assembled into parasporal crystals during sporulation. After ingestion, these crystals dissolve in the midgut and active toxins with molecular masses of about 65-kDa are released from the N-terminal half of the molecule by midgut proteases. Direct synthesis of the toxin-containing N-terminal half of Cry1 molecules using recombinant DNA techniques results in a low level of unstable truncated proteins that do not crystallize. In the present study, inclusions of truncated Cry1C (Cry1C-t) were obtained by combining genetic elements from other endotoxin genes and operons that enhance Cry protein synthesis and crystallization. Increased levels of Cry1C-t synthesis were achieved by using cyt1A promoters to drive expression of the 5' half of cry1C that included in the construct the 5' cry3A STAB-SD mRNA stabilizing sequence and the 3' stem-loop transcription terminator. RNA dot blot analysis showed that the STAB-SD and 3' transcriptional termination sequences were important for stabilization of truncated cry1C (cry1C-t) mRNA. A low level of cry1C-t mRNA was present when only the cyt1A promoters were used to express cry1C-t, but no accumulation of Cry1C-t was detected in Western blots. The orientation of the transcription terminator was important to enhancing Cry1C-t synthesis. Inclusion of the 20- and 29-kDa helper protein genes in cry1C-t constructs further enhanced synthesis. The Cry1C-t protein was toxic to Spodoptera exigua larvae, though the toxicity (50% lethal concentration [LC(50)] = 13.2 microg/ml) was lower than that of full-length Cry1C (LC(50) = 1.8 microg/ml). However, transformation of the HD1 isolate of B. thuringiensis subsp. kurstaki with the cry1C-t construct enhanced its toxicity to S. exigua as much as fourfold.

Identification and purification of the 69-kDa intracellular protease involved in the proteolytic processing of the crystal delta-endotoxin of Bacillus thuringiensis subsp. tenebrionis

The dynamics of appearance of intracellular proteases in relation to the synthesis of crystal delta-endotoxin was studied to identify the native intracellular protease(s) involved in the proteolytic processing of the 73-kDa protoxin of Bacillus thuringiensis subsp. tenebrionis. In vitro proteolytic activation of the 73-kDa protoxin indicated the possible role of 69-kDa protease in the proteolytic processing of 73-kDa protoxin. The purified 69-kDa protease was able to cause the proteolytic activation of the 73-kDa protoxin to 68-kDa toxin and this conversion was inhibited by ethylenediamine tetraacetic acid and 1,10-phenanthroline.

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

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

Revision of the nomenclature for the Bacillus thuringiensis pesticidal crystal proteins

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

Differential effects of helper proteins encoded by the cry2A and cry11A operons on the formation of Cry2A inclusions in Bacillus thuringiensis

To compare the differential effects of cry2A operon orf2 (29-kDa protein gene) and Cry11A operon orf3 (20-kDa protein gene) on Cry2A synthesis and inclusion formation, we expressed the cry2A gene along with either the 29-kDa gene, 20-kDa gene, or both genes. Constructs containing 20-kDa, in the presence or absence of 29-kDa, produced more Cry2A than constructs which lacked this gene. Cry2A synthesis was also higher when the 29-kDa gene was included with 20-kDa in the construct. However, even in the presence of increased Cry2A synthesis facilitated by the 20-kDa gene, typical Cry2A crystals did not form if the 29-kDa gene was not included in the construct. These results suggest that the 29-kDa and 20-kDa proteins have different functions, with the 20-kDa protein acting like a molecular chaperone to enhance net Cry2A synthesis, and the 29-kDa protein likely serving as a template for the stabilization of Cry2A molecules and their organization into the rectangular inclusion characteristic of wild-type Cry2A crystals.

PCR-based approach for detection of novel Bacillus thuringiensis cry genes

A two-step strategy, named exclusive PCR or E-PCR, has been developed to overcome the main limitation of PCR, which is the detection of already-known sequences only. This strategy allows the ability to detect and further clone and sequence genes for which no specific primers are available and in which a variable region exists between two conserved regions. This approach has been applied to Bacillus thuringiensis cryI genes by the use of mixtures of degenerate and specific primers recognizing well-known sequences. The first step allows the accurate identification of already-characterized cryI genes by the use of three primers. During the second step, the same sets of primers are used to exclude known sequences and to positively detect cryI genes unrecognized by any specific primer. The method, as well as its application to detect, clone, and sequence a novel cryIB gene, is described in this article.

Identification and characterization of a previously undescribed cyt gene in Bacillus thuringiensis subsp. israelensis

Mosquitocidal Bacillus thuringiensis strains show as a common feature the presence of toxic proteins with cytolytic and hemolytic activities, Cyt1Aa1 being the characteristic cytolytic toxin of Bacillus thuringiensis subsp. israelensis. We have detected the presence of another cyt gene in this subspecies, highly homologous to cyt2An1, coding for the 29-kDa cytolytic toxin from B. thuringiensis subsp. kyushuensis. This gene, designated cyt2Ba1, maps upstream of cry4B coding for the 130-kDa crystal toxin, on the 72-MDa plasmid of strain 4Q2-72. Sequence analysis revealed, as a remarkable feature, a 5' mRNA stabilizing region similar to those described for some cry genes. PCR amplification and Southern analysis confirmed the presence of this gene in other mosquitocidal subspecies. Interestingly, anticoleopteran B. thuringiensis subsp. tenebrionis belonging to the morrisoni serovar also showed this gene. On the other hand, negative results were obtained with the anti-lepidopteran strains B. thuringiensis subsp. kurstaki HD-1 and subsp. aizawai HD-137. Western analysis failed to reveal Cyt2A-related polypeptides in B. thuringiensis subsp. israelensis 4Q2-72. However, B. thuringiensis subsp. israelensis 1884 and B. thuringiensis subsp. tenebrionis did show cross-reactive products, although in very small amounts.

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

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

Overproduction of encapsulated insecticidal crystal proteins in a Bacillus thuringiensis spo0A mutant

The spo0A gene of Bacillus subtilis encodes the key factor involved in the initiation of sporulation. It was previously shown that the B. thuringiensis (Bt) cryIIIA gene, encoding a toxin active against coleopteran larvae, is overexpressed in an spo0A mutant of B. subtilis. In this paper we describe the construction of a Bt spo0A mutant strain and its use to produce insecticidal crystal proteins. The spo0A gene of Bt was cloned and identified by its ability to transform a B. subtilis spo0A mutant to prototrophy. Its nucleotide sequence is homologous to the B. subtilis gene. The spo0A gene was replaced in the Bt genome with a disrupted copy to give an Spo- strain unable to initiate sporulation. When the cryIIIA gene was cloned in the Bt spo0A mutant, large amounts of toxins were produced and accumulated to form a large crystal inclusion which remained encapsulated within the ghost cell. These encapsulated toxins were highly active against coleopteran larvae. We anticipate that the cryIIIA expression system and the Bt spo0A mutant will provide a convenient process to generate novel formulations of stabilized and environmentally safe Bt-based biopesticides.

Elucidation of the mechanism of CryIIIA overproduction in a mutagenized strain of Bacillus thuringiensis var. tenebrionis

NB176 is a Bacillus thuringiensis mutant derived by gamma-irradiation of NB125 Bacillus thuringiensis var. tenebrionis (Krieg). It exhibits two interesting phenotypes: (i) oligosporogeny and (ii) twofold to threefold overproduction of the CryIIIA protein. Southern profiles of the NB176 strain showed an additional copy(s) of the cryIIIA gene located on a 4 kb HindIII fragment, in addition to the expected cryIIIA gene on a 3 kb HindIII fragment. Each cryIIIA gene-bearing HindIII fragment was cloned from NB176. The restriction map of the 3 kb HindIII fragment was identical to that published by Donovan and coworkers. Sequencing of the 4 kb HindIII fragment showed no alterations in the promoter region of the cryIIIA gene but did show replacement of the region immediately following the cryIIIA open reading frame with a sequence encoding a transposase with 50% amino acid homology to that of Tn1000. These findings suggest that the overproduction phenotype of NB176 results from extra copies of the cryIIIA gene produced from a transposition event(s) induced or stabilized by gamma-irradiation. Integration of additional copies of the cryIIIA gene into the native 90 MDa plasmid of the wild-type B. thuringiensis var. tenebrionis strain resulted in strains that made enormous crystals, many possessing greatly enhanced insecticidal activity.

Expression in Bacillus subtilis of the Bacillus thuringiensis cryIIIA toxin gene is not dependent on a sporulation-specific sigma factor and is increased in a spo0A mutant

Expression of the Bacillus thuringiensis cryIIIA gene encoding a Coleoptera-specific toxin is weak during vegetative growth and is activated at the onset of the stationary phase. cryIIIA'-'lacZ fusions and primer extension analysis show that the regulation of cryIIIA expression is similar in Bacillus subtilis and in B. thuringiensis. Activation of cryIIIA expression was not altered in B. subtilis mutant strains deficient for the sigma H and sigma E sporulation-specific sigma factors or for minor sigma factors such as sigma B, sigma D, or sigma L. This result and the nucleotide sequence of the -35 and -10 regions of the cryIIIA promoter suggest that cryIIIA expression might be directed by the E sigma A form of RNA polymerase. Expression of the cryIIIA'-'lacZ fusion is shut off after t2 (2 h after time zero) of sporulation in the B. subtilis wild-type strain grown on nutrient broth sporulation medium. However, no decrease in cryIIIA-directed beta-galactosidase activity occurred in sigma H, kinA, or spo0A mutant strains. Moreover, beta-galactosidase activity was higher and remained elevated after t2 in the spo0A mutant strain. beta-Galactosidase activity was weak in abrB and spo0A abrB mutant strains, suggesting that AbrB is responsible for the higher level of cryIIIA expression observed in a spo0A mutant. However, both in spo0A and spo0A abrB mutant strains, beta-galactosidase activity remained elevated after t2, suggesting that even in the absence of AbrB, cryIIIA expression is controlled through modulation of the phosphorylated form of Spo0A. When the cryIIIA gene is expressed in a B. subtilis spo0A mutant strain or in the 168 wild-type strain, large amounts of toxins are produced and accumulate to form a flat rectangular crystal characteristic of the coleopteran-specific B. thuringiensis strains.

Tn5401 disruption of the spo0F gene, identified by direct chromosomal sequencing, results in CryIIIA overproduction in Bacillus thuringiensis

The Bacillus thuringiensis spo0F gene was identified by chromosomal DNA sequencing of sporulation mutants derived from a B. thuringiensis transposon insertion library. A spo0F defect in B. thuringiensis, which was suppressed by multicopy hknA or kinA, resulted in the overproduction of the CryIIIA insecticidal crystal protein.

Overexpression of Bacillus thuringiensis HknA, a histidine protein kinase homology, bypasses early Spo mutations that result in CryIIIA overproduction

The Bacillus thuringiensis CryIIIA insecticidal crystal protein (ICP) is a vegetatively expressed protein that is toxic to coleopteran insect larvae. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the asporogenous B. thuringiensis subsp. morrisoni strain EG1351, which harbors the native cryIIIA-encoding 88-MDa plasmid, showed a 2.5-fold overproduction of the CryIIIA protein compared with that of an isogenic wild-type strain. Further studies showed that neither CryIIIA protein synthesis nor CryIIIA protein processing was affected in strain EG1351 during vegetative growth. In an attempt to characterize the EG1351 mutation by complementation of function, the hknA gene was identified and cloned from a B. thuringiensis cosmid library. Primer extension analysis of hknA mRNA in wild-type B. thuringiensis demonstrated that the hknA gene is transcribed during vegetative growth from a sigma A-like promoter. Multiple copies of either the hknA gene or the Bacillus subtilis kinA (spoIIJ) gene were shown to bypass the sporulation defect in strain EG1351 as well as a spo0F mutation in B. thuringiensis EG1634. Additional studies showed that the hknA gene was not defective in strain EG1351. The results of this study suggest that hknA encodes a novel histidine protein kinase involved in B. thuringiensis sporulation. We also propose that the CryIIIA-overproducing phenotype of strain EG1351 is most likely due to a defect in the phosphorylation of Spo0A and confirm that CryIIIA production is not dependent on sporulation.

Structural and functional analysis of the promoter region involved in full expression of the cryIIIA toxin gene of Bacillus thuringiensis

The promoter region of the cryIIIA toxin gene of Bacillus thuringiensis is composed of at least three domains: an upstream region extending from nucleotide positions -635 to -553 (with reference to the translational start codon of cryIIIA), an internal region extending from nucleotide positions -553 to -367, and a downstream region extending from nucleotide position -367 to +18. Deletion analysis and transcriptional fusions to the lacZ gene indicate that full expression of cryIIIA requires the association of the upstream and the downstream region. Primer extension experiments reveal a major cryIIIA transcript (designated T-129) starting at nucleotide position -129 and another transcript (designated T-558) starting at nucleotide position -558. Mutation in the -35 region of the promoter responsible for the initiation of T-558 indicates that the upstream promoter is essential for full expression of cryIIIA, although not sufficient. Deletion of the DNA region carrying the previously described cryIIIA promoter does not affect full expression of cryIIIA and does not modify the 5' end of T-129. Taken together, these results indicate that the 5' end of T-129 is not a trnascriptional start site. Therefore, we propose that T-129 results from the processing of the mRNA initiated at the upstream promoter (T-558), generating a stable mRNA with a 5' extremity at nucleotide position -129. From primer extension analysis and transcriptional fusions to lacZ, it appears that the upstream promoter is weakly but significantly expressed during the vegetative phase of growth, is activated at the onset of sporulation and remains active at least until t5. However, unlike the promoters of other cry genes, this promoter is similar to sigma A-dependent promoters rather than sporulation-specific promoters. This promoter may therefore be transcribed by the E sigma A form of RNA polymerase. Activation at the onset of sporulation could result from the disappearance of a repressor, or the appearance of a stationary-phase-specific activator.

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

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

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

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

Genetically improved potatoes: protection from damage by Colorado potato beetles

Russet Burbank potato plants have been genetically improved to resist insect attack and damage by Colorado potato beetles (Leptinotarsa decemlineata (Say)) by the insertion of a cryIIIA gene encoding the insect control protein of Bacillus thuringiensis var. tenebrionis. A modified gene that dramatically improved plant expression of this protein was utilized. Its expression in Russet Burbank potato plants resulted in protection from damage by all insect stages in the laboratory and in dramatic levels of protection at multiple field locations. Analysis of these genetically modified potatoes indicated that they conform to the standards for Russet Burbank potatoes in terms of agronomic and quality characteristics including taste.

Full expression of the cryIIIA toxin gene of Bacillus thuringiensis requires a distant upstream DNA sequence affecting transcription

The cryIIIA gene encoding a coleopteran-specific toxin is poorly expressed in Bacillus thuringiensis when cloned in a low-copy-number plasmid. This weak expression is observed when the gene is cloned only with its promoter and its putative terminator. cryIIIA gene expression was analyzed by using deletion derivatives of a larger DNA fragment carrying the toxin gene and additional adjacent sequences. The results indicate that a 1-kb DNA fragment located 400 bp upstream of the promoter strongly enhances CryIIIA production in B. thuringiensis sporulating cells. Similar results were obtained when the low-copy-number plasmid pHT304 carrying transcriptional fusions between upstream regions of cryIIIA and the lacZ gene was used. Analysis of the start sites, the sizes, and the amounts of cryIIIA-specific mRNAs shows that the enhancement occurs at the transcriptional level by increasing the number of cryIIIA-specific transcripts from the onset of sporulation to about 6 h after the onset of sporulation. The nucleotide sequence of the 1-kb activating fragment and of the 700 bp containing the promoter region and the 5' end of cryIIIA were determined. No potential protein-coding sequences were found upstream of the promoter. The major characteristic of the 1-kb activating fragment is the presence of a 220-bp A + T-rich region.

The reconstruction and expression of a Bacillus thuringiensis cryIIIA gene in protoplasts and potato plants

A Bacillus thuringiensis (B.t.) cryIIIA delta-endotoxin gene was designed for optimal expression in plants. The modified cry gene has the codon usage pattern of an average dicot gene and does not contain AT-rich nucleotide sequences typical of native B.t. cry genes. We assembled the 1.8 kb cryIIIA gene in nine blocks of three oligonucleotide pairs. For two DNA blocks, the polymerase chain reaction was used to enrich for correctly ligated pairs. We compared modified cryIIIA gene with native gene expression by electroporation of dicot (carrot) and monocot (corn) protoplasts. CryIIIA-specific RNA and protein was detected in carrot and corn protoplasts only after electroporation with the rebuilt gene. Transgenic potato lines were generated containing the redesigned cryIIIA gene under the transcriptional control of a chimeric CaMV 35S/mannopine synthetase (Mac) promoter. Out of 63 transgenic potato lines, 58 controlled first-instar Colorado potato beetle (CPB) larvae in bioassays. Egg masses which produced ca. 250,000 CPB larvae were placed on replicate clones of 56 transgenic potatoes. No CPB larvae developed past the second instar on any of these plants. Plants expressing high levels of delta-endotoxin were identified by their toxicity to more resistant third-instar larvae. We show there was good correlation between insect control and the levels of delta-endotoxin RNA and protein.

Efficient splicing of an AU-rich antisense intron sequence

For successful splicing in dicot plants the only recognised intron requirements are 5' and 3' splice sites and AU-rich sequences. We have investigated further the importance of AU-rich elements by analyzing the splicing of an AU-rich antisense intron sequence. Activation of cryptic splice sites on either side of the AU-rich sequence permitted the efficient removal of this essentially non-intron sequence by splicing. This splicing event not only confirms the importance of AU-rich sequences but also has implications for the evolution of interrupted genes and the expression of heterologous genes in transgenic plants.

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

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

Expansion of insecticidal host range of Bacillus thuringiensis by in vivo genetic recombination

We describe a novel approach for the insertion of an insecticidal toxin gene into a resident plasmid in Bacillus thuringiensis (Bt). A gene encoding a coleopteran-specific toxin was cloned within a fragment of IS232 and inserted into a plasmid thermosensitive for replication in Bt. The plasmid was used to transform a Bt strain toxic to lepidoptera, and the transformants were then selected at non-permissive temperature for clones in which the vector had integrated into a copy of IS232 present on a resident plasmid. A second recombination event was selected such that the vector was eliminated and the newly introduced toxin gene was conserved. The resulting strain contained only DNA of Bt origin, and displayed insecticidal activity against both lepidoptera and coleoptera.

Synthetic cryIIIA gene from Bacillus thuringiensis improved for high expression in plants

A 1974 bp synthetic gene was constructed from chemically synthesized oligonucleotides in order to improve transgenic protein expression of the cryIIIA gene from Bacillus thuringiensis var. tenebrionis in transgenic tobacco. The crystal toxin genes (cry) from B. thuringiensis are difficult to express in plants even when under the control of efficient plant regulatory sequences. We identified and eliminated five classes of sequence found throughout the cryIIIA gene that mimic eukaryotic processing signals and which may be responsible for the low levels of transcription and translation. Furthermore, the GC content of the gene was raised from 36% to 49% and the codon usage was changed to be more plant-like. When the synthetic gene was placed behind the cauliflower mosaic virus 35S promoter and the alfalfa mosaic virus translational enhancer, up to 0.6% of the total protein in transgenic tobacco plants was cryIIIA as measured from immunoblot analysis. Bioassay data using potato beetle larvae confirmed this estimate.

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

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

Identification and characterization of a novel Bacillus thuringiensis delta-endotoxin entomocidal to coleopteran and lepidopteran larvae

A new class of Bacillus thuringiensis delta-endotoxins, or insecticidal control proteins (ICPs), is defined by an apparently cryptic protein with a unique primary structure and novel entomocidal specificity for certain coleopteran and lepidopteran species. The discovery of a new group of ICPs will extend the use of this natural insecticide in integrated pest-management systems.

Two novel strains of Bacillus thuringiensis toxic to coleopterans

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

Crystal structure of insecticidal delta-endotoxin from Bacillus thuringiensis at 2.5 A resolution

The structure of the delta-endotoxin from Bacillus thuringiensis subsp. tenebrionis that is specifically toxic to Coleoptera insects (beetle toxin) has been determined at 2.5 A resolution. It comprises three domains which are, from the N- to C-termini, a seven-helix bundle, a three-sheet domain, and a beta sandwich. The core of the molecule encompassing all the domain interfaces is built from conserved sequence segments of the active delta-endotoxins. Therefore the structure represents the general fold of this family of insecticidal proteins. The bundle of long, hydrophobic and amphipathic helices is equipped for pore formation in the insect membrane, and regions of the three-sheet domain are probably responsible for receptor binding.

Analysis of unstable RNA transcripts of insecticidal crystal protein genes of Bacillus thuringiensis in transgenic plants and electroporated protoplasts

We have examined expression of several insecticidal crystal protein (ICP) genes of Bacillus thuringiensis in transgenic tobacco plants and electroporated carrot protoplasts. We determined that low levels of lepidopteran toxin cryIA(b) ICP gene expression in plants and electroporated carrot cells is due to RNA instability. We used a series of 3' deleted by cryIA(b) constructs directed by the cauliflower mosaic virus 35S promoter to demonstrate that this instability is minimally contained in the first 579 bases of the gene in both systems. This instability may result from 5'----3' as well as 3'----5' RNA metabolism. The coleopteran toxic cryIIIA gene was also examined in electroporated carrot cells, and found to be poorly expressed. A model for improvement of ICP RNA stability in plants is presented.

Activation of a cryptic crystal protein gene of Bacillus thuringiensis subspecies kurstaki by gene fusion and determination of the crystal protein insecticidal specificity

DNA hybridization with the insecticidal crystal protein gene cryllA (formerly cryBl) of Bacillus thuringiensis supspecies kurstaki has shown that subspecies kurstaki contains a cryllA-related sequence in addition to the cryllA gene (Donovan et al., 1988a). We have cloned the cryllA-related sequence and have determined that the sequence, which has been designated cryllB, is 89% identical to the cryllA gene. Recombinant B. thuringiensis cells harbouring the cloned cryllB gene produced very little CryllB protein. A high level of production of the CryllB protein was achieved by fusing the regulatory region of the crylllA crystal protein gene to the cryllB gene. The CryllB protein was found to be highly toxic to Lymantria dispar, Heliothis virescens and Trichoplusia ni, and was not toxic to Aedes aegypti.

The construction of Bacillus thuringiensis strains expressing novel entomocidal delta-endotoxin combinations

Using our recently reported method of electroporation to transform Bacillus thuringiensis [Bone & Ellar (1989) FEMS Microbiol. Lett. 58, 171-178], cloned B. thuringiensis entomocidal delta-endotoxin genes have been introduced into several native B. thuringiensis strains. In many cases the resulting transformants expressed both their native toxins and the cloned toxin, producing strains with broader toxicity spectra. The introduction of the var. tenebrionis toxin gene into B. thuringiensis var. israelensis resulted in a strain with activity against Pieris brassicae (cabbage white butterfly), an activity which neither parent strain possesses. We discuss further the possibility of synergism and also the problems associated with introducing cloned DNA by this method.