A novel Bacillus thuringiensis gene encoding a Spodoptera exigua-specific crystal protein

Isolation and molecular characterization of Bacillus thuringiensis subsp. kurstaki toxic to lepidopteran pests Spodoptera spp. and Plutella xylostella

BACKGROUND

Bacillus thuringiensis (Bt) has been widely used as a biological control agent for lepidopteran pests. However, resistance to Bt is a major concern associated with Spodoptera spp. (Noctuidae) and Plutella xylostella (Plutellidae). For efficient control of Noctuidae and Plutellidae, novel Bt strains which have high toxicity and a broad host range are needed.

RESULTS

To develop novel Bt strains as used for bio-insecticides, the Bt IMBL-B9 with high toxicity against Spodoptera exigua, Spodoptera frugiperda and P. xylostella was isolated and characterized. The Bt kurstaki IMBL-B9 strain produced bipyramidal and cuboidal crystals consisting of cry toxins with molecular weights of 130 and 65 kDa, respectively. This strain harbors eight crystal protein genes in total, including cry1Ea and one vegetative insecticidal protein gene. The median lethal concentration (LC₅₀ ) values of IMBL-B9 against S. exigua and S. frugiperda were 21.8- and 19.3-fold lower than those of the Bt kusrstaki strain, and 5.6- and 4.9-fold lower than those of Bt aizawai strain, respectively. To evaluate the insecticidal activity of Cry proteins from IMBL-B9, cry gene-sourced recombinant Bt strains were constructed. These strains have insecticidal activity and synergic action against lepidopteran pests.

CONCLUSION

In this study, a novel Bt kurstaki IMBL-B9 strain was isolated and this could be useful for the development of new bio-insecticide or cry gene-based recombinant products as an alternative solution against lepidopterans, including Noctuidae and Plutellidae. © 2022 Society of Chemical Industry.

Nematicidal Activity of Cry1Ea11 from Bacillus thuringiensis BRC-XQ12 Against the Pine Wood Nematode (Bursaphelenchus xylophilus)

The nematicidal activity of 92 Bacillus thuringiensis strains against the pine wood nematode Bursaphelenchus xylophilus, one of the world's top 10 plant-parasitic nematodes, was determined. The insecticidal crystal proteins (ICPs) from Bacillus thuringiensis BRC-XQ12 were the most toxic to Bursaphelenchus xylophilus, with a lethal concentration 50 (LC₅₀) of 32.13 μg/ml. Because the ICPs expressed by Bacillus thuringiensis BRC-XQ12 were closest to Cry1Ea6 and B. thuringiensis BRC-XQ12 contained four kinds of cry1 subgenes (cry1Aa, cry1Cb, cry1Ea, and cry1Ia), Cry1Ea was most likely to be the key active component against the nematode. The 3,516-bp cry1Ea11 gene from BRC-XQ12, as designated by the B. thuringiensis δ-endotoxin nomenclature committee, was expressed in Escherichia coli. Purified Cry1Ea11 showed an LC₅₀ of 32.53 and 23.23 μg/ml at 24 and 48 h, with corresponding virulence equations of Y = 32.15X + 1.38 (R² = 0.9951) and Y = 34.29X + 3.16 (R² = 0.9792), respectively. In order to detect the pathway of B. thuringiensis Cry1Ea11 into Bursaphelenchus xylophilus, the nematode was fed with NHS-rhodamine-labeled GST-Cry1Ea11. The results of confocal laser-scanning microscopy showed that the 159-kDa GST-Cry1Ea11 could be detected in the stylet and the esophageal lumen of the pine wood nematode, indicating that GST-Cry1Ea11 could enter into the nematode through the stylet. As far as we know, no Cry1 proteins have been shown to have activity against plant-parasitic nematodes before. These results demonstrate that Cry1Ea11 is a promising nematicidal protein for controlling pine wilt disease rendered by B. xylophilus, further dramatically broadening the spectrum of Bacillus thuringiensis ICPs.

Construction and characterisation of an antifungal recombinant Bacillus thuringiensis with an expanded host spectrum

A novel antifungal Bacillus thuringiensis strain 19-22, ssp. kurstaki (H3a3b3c), was characterised. This strain included cry1Aa, cry1Ab, cry1Ac, and cry1D, which have high insecticidal activities against lepidopteran larvae other than Spodoptera exigua. To expand the host spectrum, a cry1E gene whose product is active against S. exigua was introduced into the isolate. The transformant successfully expressed the Cry1E protein without any loss of its original antifungal activities. These results indicate that this recombinant strain exhibits dual activities and may be used as an integrated control agent to control plant diseases and insect pests.

Transgenic rice plants expressing a modified cry1Ca1 gene are resistant to Spodoptera litura and Chilo suppressalis

Nucleotide sequence encoding the truncated insecticidal Cry1Ca1 protein from Bacillus thuringiensis was extensively modified based on the codon usage of rice genes. The overall G + C contents of the synthetic cry1Ca1 coding sequence were raised to 65% with an additional bias of enriching for G and C ending codons as preferred by monocots. The synthetic gene was introduced into the Chinese japonica variety, Xiushui 11, by Agrobacterium-mediated transformation. Transgenic rice plants harboring this gene were highly resistant to Chilo suppressalis and Spodoptera litura larvae as revealed by insect bioassays. High levels of Cry1Ca1 protein were obtained in the leaves of transgenic rice, which were effective in achieving 100% mortality of S. litura and C. suppressalis larvae. The levels of Cry1Ca1 expression in the leaves of these transgenic plants were up to 0.34% of the total soluble proteins. The larvae of C. suppressalis and S. litura could consume a maximum of 1.89 and 4.89 mm2 of transgenic leaf area whereas the consumption of nontransgenic leaves by these larvae was significantly higher; 58.33 and 61.22 mm2, respectively. Analysis of R1 transgenic plants indicated that the cry1Ca1 was inherited by the progeny plants and provided complete protection against C. suppressalis and S. litura larvae.

Population dynamics of Spodoptera litura (Lepidoptera: Noctuidae) on Bt cotton in the Yangtze River Valley of China

Genetically modified cotton that produces a crystalline protein from Bacillus thuringiensis subsp. kurstaki (Berliner) (Bt) has been widely deployed to manage lepidopteran insect pests in cotton growing areas worldwide. However, susceptibility of different insect species to Bt protein varies, which may affect lepidopteran pest populations in the field. Studies on effects of two transgenic cotton lines (BG1560 and GK19) carrying a Cry1A gene on common cutworm Spodoptera litura F. (Lepidoptera: Noctuidae), were conducted during 2002-2005 in the cotton planting region of the Yangtze River valley of China. Results showed that common cutworm larvae had low susceptibility to Bt cotton. There was no significant difference in larval population densities in conventional and Bt cotton fields. However, the larval populations of the insect on conventional plants treated with chemical insecticides for control of target pest of Bt cotton were significantly lower than that in Bt cotton fields. These results indicated that the common cutworm was the potential to become a major and alarming pest in Bt cotton fields, and therefore efforts to develop an effective alternative management strategy are needed.

Protease inhibitors fail to prevent pore formation by the activated Bacillus thuringiensis toxin Cry1Aa in insect brush border membrane vesicles

To investigate whether membrane proteases are involved in the activity of Bacillus thuringiensis insecticidal toxins, the rate of pore formation by trypsin-activated Cry1Aa was monitored in the presence of a variety of protease inhibitors with Manduca sexta midgut brush border membrane vesicles and by a light-scattering assay. Most of the inhibitors tested had no effect on the pore-forming ability of the toxin. However, phenylmethylsulfonyl fluoride, a serine protease inhibitor, promoted pore formation, although this stimulation only occurred at higher inhibitor concentrations than those commonly used to inhibit proteases. Among the metalloprotease inhibitors, o-phenanthroline had no significant effect; EDTA and EGTA reduced the rate of pore formation at pH 10.5, but only EDTA was inhibitory at pH 7.5. Neither chelator affected the properties of the pores already formed after incubation of the vesicles with the toxin. Taken together, these results indicate that, once activated, Cry1Aa is completely functional and does not require further proteolysis. The effect of EDTA and EGTA is probably better explained by their ability to chelate divalent cations that could be necessary for the stability of the toxin's receptors or involved elsewhere in the mechanism of pore formation.

Identification of cry-Type Genes on 20-kb DNA Associated with Cry1 Crystal Proteins from Bacillus thuringiensis

Heterologous DNA fragments (20-kb) associated with Cry1 crystal proteins (protoxins) from a soil-isolated Bacillus thuringiensis strain 4.0718 were isolated and analyzed. RFLP patterns of the PCR products showed that the 20-kb DNA fragments harbored cry1Aa, cry1Ac, cry2Aa, and cry2Ab genes. Furthermore, a 4.2-kb DNA fragment, which contained the promoter, the coding region, and the terminator of cry1Ac gene, was cloned from the 20-kb DNAs by PCR, and then the cry1Ac gene was expressed in an acrystalliferous B. thuringiensis strain 4Q7 by using E. coli-B. thuringiensis shuttle vector pHT3101. SDS-PAGE and microscopy studies revealed that the recombinant could express 130-kDa Cry1Ac protoxin and produce bipyramidal crystals during sporulation. Bioassay results proved that crystal-spore mixture from the recombinant was toxic to Plutella xylostella. This was the first report of cry-type genes present on 20-kb DNA associated with Cry1 protoxins of B. thuringiensis.

Development of a hybrid delta-endotoxin and its expression in tobacco and cotton for control of a polyphagous pest Spodoptera litura

A hybrid delta-endotoxin protein was designed against a polyphagous lepidopteran insect pest Spodoptera litura, which is tolerant to most of the known delta-endotoxins. The hybrid delta-endotoxin was created by replacing amino acid residues 530-587 in a poorly active natural Cry1Ea protein, with a highly homologous 70 amino acid region of Cry1Ca in domain III. The truncated delta-endotoxins Cry1Ea, Cry1Ca and the hybrid protein Cry1EC accumulated in Escherichia coli to form inclusion bodies. The solubilised Cry1EC made from E. coli was 4- fold more toxic to the larvae of S. litura than Cry1Ca, the best known delta-endotoxin against Spodoptera sp. None of the two truncated toxins, solubilised from E. coli caused larval mortality. However, trypsinised Cry1Ca protoxin obtained from E. coli and solubilised from inclusion bodies caused mortality of S. litura with LC50 513 ng/ml semi synthetic diet. A synthetic gene coding for the hybrid delta-endotoxin Cry1EC was designed for high level expression in plants, taking into consideration several features found in the highly expressed plant genes. Transgenic, single copy plants of tobacco as well as cotton were developed. The selected lines expressed Cry1EC at 0.1-0.7% of soluble leaf protein. Such plants were completely resistant to S. litura and caused 100% mortality in all stages of larval development. Hence, unlike in E. coli, the hybrid delta-endotoxin folded into a functionally active conformation in both tobacco and cotton leaves. The truncated Cry1EC expressed in tobacco leaves was about 8-fold more toxic (LC50 58 ng/ml diet) compared to expression in E. coli.

Interaction between functional domains of Bacillus thuringiensis insecticidal crystal proteins

Interactions among the three structural domains of Bacillus thuringiensis Cry1 toxins were investigated by functional analysis of chimeric proteins. Hybrid genes were prepared by exchanging the regions coding for either domain I or domain III among Cry1Ab, Cry1Ac, Cry1C, and Cry1E. The activity of the purified trypsin-activated chimeric toxins was evaluated by testing their effects on the viability and plasma membrane permeability of Sf9 cells. Among the parental toxins, only Cry1C was active against these cells and only chimeras possessing domain II from Cry1C were functional. Combination of domain I from Cry1E with domains II and III from Cry1C, however, resulted in an inactive toxin, indicating that domain II from an active toxin is necessary, but not sufficient, for activity. Pores formed by chimeric toxins in which domain I was from Cry1Ab or Cry1Ac were slightly smaller than those formed by toxins in which domain I was from Cry1C. The properties of the pores formed by the chimeras are therefore likely to result from an interaction between domain I and domain II or III. Domain III appears to modulate the activity of the chimeric toxins: combination of domain III from Cry1Ab with domains I and II of Cry1C gave a protein which was more strongly active than Cry1C.

Characterization of cry genes in a Mexican Bacillus thuringiensis strain collection

Mexico is located in a transition zone between the Nearctic and Neotropical biogeographical regions and contains a rich and unique biodiversity. A total of 496 Bacillus thuringiensis strains were isolated from 503 soil samples collected from the five macroregions of the country. The characterization of the strain collection provided useful information on the ecological patterns of distribution of B. thuringiensis and opportunities for the selection of strains to develop novel bioinsecticidal products. The analysis of the strains was based on multiplex PCR with novel general and specific primers that could detect the cry1, cry3, cry5, cry7, cry8, cry9, cry11, cry12, cry13, cry14, cry21, and cyt genes. The proteins belonging to the Cry1 and Cry9 groups are toxic for lepidopteran insects. The Cry3, Cry7, and Cry8 proteins are active against coleopteran insects. The Cry5, Cry12, Cry13, and Cry14 proteins are nematocidal. The Cry11, Cry21, and Cyt proteins are toxic for dipteran insects. Six pairs of general primers are used in this method. Strains for which unique PCR product profiles were obtained with the general primers were further characterized by additional PCRs with specific primers. Strains containing cry1 genes were the most abundant in our collection (49.5%). Thirty-three different cry1-type profiles were identified. B. thuringiensis strains harboring cry3 genes represented 21.5% of the strains, and 7.9% of the strains contained cry11 and cyt genes. cry7, cry8, and cry9 genes were found in 0.6, 2.4, and 2.6% of the strains, respectively. No strains carrying cry5, cry12, cry13, cry14, or cry21 genes were found. Finally, 14% of the strains did not give any PCR product and did not react with any polyclonal antisera. Our results indicate the presence of strains that may harbor potentially novel Cry proteins as well as strains with combinations of less frequently observed cry genes.

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.

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.

Synergistic activity of a Bacillus thuringiensis delta-endotoxin and a bacterial endochitinase against Spodoptera littoralis larvae

In an attempt to increase the insecticidal effect of the delta-endotoxin crystal protein CryIC on the relatively Cry-insensitive larvae of Spodoptera littoralis, a combination of CryIC and endochitinase was used. CryIC comprising the first 756 amino acids from Bacillus thuringiensis K26-21 and endochitinase ChiAII encoded by Serratia marcescens were separately produced in Escherichia coli carrying the genes in overexpression vectors. The endochitinase on its own, even at very low concentrations (0.1 microgram/ml), perforated the larval midgut peritrophic membrane. When applied together with low concentrations of CryIC, a synergistic toxic effect was obtained. In the absence of chitinase, about 20 micrograms of CryIC per ml was required to obtain maximal reduction in larval weight, while only 3.0 micrograms of CryIC per ml caused a similar toxic effect in the presence of endochitinase. Thus, a combination of the Cry protein and an endochitinase could result in effective insect control in transgenic systems in which the Cry protein is not expressed in a crystalline form.

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

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

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

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

Insect resistance of transgenic plants that express modified Bacillus thuringiensis cryIA(b) and cryIC genes: a resistance management strategy

Tobacco and tomato plants were generated exhibiting insect resistance due to the introduction of modified cryIA(b) and cryIC genes of Bacillus thuringiensis. Limited modifications at selected regions of the coding sequences of both genes are sufficient to obtain resistance against Spodoptera exigua, Heliothis virescens and Manduca sexta. The criteria used to modify both genes demonstrate that the removal of sequence motifs potentially resulting in premature polyadenylation and transcript instability causes increased insect resistance. The expression of a cryIC-cryIA(b) fusion resulting in protection against S. exigua, H. virescens and M. sexta demonstrates the potential of expressing translational fusions, not only to broaden the insect resistance of transgenic plants, but also to simultaneously employ different gene classes in resistance management strategies.

Cytolytic differences among lepidopteran cell lines exposed to toxins of Bacillus thuringiensis subsp. kurstaki (HD-263) and aizawai (HD-112): effect of aminosugars and N-glycosylation

Comparison of lytic-dose response behavior of seven lepidopteran cell lines to the activated delta-endotoxin polypeptides of Bacillus thuringiensis subspecies kurstaki (HD-263) and aizawai (HD-112) indicated distinct differences among the lines. The lines derived from Spodoptera species S. exigua (URC-SE-1A) and S. littoralis (UIV-SL-575) were more susceptible to lysis by aizawai toxin (Bta) than kurstaki toxin (Btk) as were cells from the Lymantria dispar line (IPLB-LD652Y). However, the concentrations of Bta required for lysis of 50% of URC-SE-1A and IPLB-LD652Y cells (LC50) were 0.2 to 0.8 micrograms/ml compared to 5 to 9 micrograms/ml for UIV-SL-575 cells. In comparison, Btk LC50 concentrations for the three lines were similar (14 to 19 micrograms/ml). Cells from S. frugiperda (IPLB-SF-21AE) and Trichoplusia ni (TN368) were similar in their response to Bta (LC50 = 2.5 to 3.7 micrograms/ml) and Btk (LC50 = 1.0 to 2.8 micrograms/ml) whereas the lines derived from Heliothis spp. were the least susceptible to both toxins. The LC50 concentrations for Bta with the H. zea line (IPLB-HA-1075) and H. virescens line (BCIRL-HV-AM1) were > 50 micrograms/ml and for Btk were > 50 micrograms/ml and 42 to 50 micrograms/ml, respectively, yet for both lines Btk was the more cytolytic. Cytolysis of TN368 cells could be inhibited to varying extents by preincubation of the toxins with the aminosugars of galactose, mannose, and glucose and their N-acetyl derivatives. The unsubstituted hexoses were not inhibitory.(ABSTRACT TRUNCATED AT 250 WORDS)

Recombinant Bacillus thuringiensis crystal proteins with new properties: possibilities for resistance management

To obtain Bacillus thuringiensis crystal proteins with new properties and to identify the regions involved in insecticidal activity, we generated hybrid genes composed of cryIC and cryIE by in vivo recombination. Analysis of the hybrid proteins showed that domain III of CryIC is involved in the toxicity towards Spodoptera exigua and Mamestra brassicae. Transfer of this domain to CryIE, which is not active against these insects, resulted in a new protein with a broader activity. This hybrid protein binds to different receptors than CryIC, suggesting its use as an alternative for CryIC in resistance management programs.

Determination and Distribution of cry -Type Genes of Bacillus thuringiensis Isolates from Taiwan

Using PCR with a set of specific oligonucleotide primers to detect cryI -type genes, we were able to screen the cry -type genes of 225 Bacillus thuringiensis soil isolates from Taiwan without much cost in time or labor. Some combinations of cry genes (the cry -type profile) in a single isolate were unique. We identified five distinct profiles of crystal genes from the B. thuringiensis soil isolates from Taiwan. The cry genes included cryIA ( a ), cryIA ( b ), cryIA ( c ), cryIC, cryID , and cryIV . Interestingly, 501 B. thuringiensis isolates (93.5% of the total number that we identified) were isolated from areas at high altitudes. The profiles of cry -type genes were distinct in all isolation areas. The distribution of cry -type genes of our isolates therefore depended on geography. Using PCR footprinting to detect cryIC -type genes, we identified two distinct cryIC footprints from some of our isolates, indicating that these isolates may contain novel cryIC -type genes. B. thuringiensis isolates containing cryIA ( a )-, cryIA ( b )-, and cryIA ( c )-type genes exhibited much greater activity against Plutella xylostella than did other isolates, indicating that multiple cry -type genes may be used as markers for the prediction of insecticidal activities.

Structural stability of Bacillus thuringiensis delta-endotoxin homolog-scanning mutants determined by susceptibility to proteases

Forty homolog-scanning (double-reciprocal-crossover) mutant proteins of two Bacillus thuringiensis delta-endotoxin genes (cryIAa and cryIAc) were examined for potential structural alterations by a series of proteolytic assays. Three groups of mutants could be identified. Group 1, consisting of 13 mutants, showed no delta-endotoxin present during overexpression conditions in Escherichia coli (48 h at 37 degrees C, with a ptac promoter). These mutants produced full-sized delta-endotoxin detectable by polyacrylamide gel electrophoresis with Coomassie blue staining or Western immunoanalysis after 24 h of growth but not after 48 h, suggesting sensitivity to intracellular proteases. Group 2 consisted of 13 mutants that produced stable delta-endotoxins that were completely digested by 2% bovine trypsin. In contrast, native delta-endotoxin produces a 65,000-Da trypsin-resistant peptide, which is the active toxin. Group 3 mutants expressed delta-endotoxin and trypsin-stable toxins, similar to the wild type. In this study, 12 group 3 mutant toxins were compared with wild type toxins by thermolysin digestion at a range of temperatures. The two wild-type toxins exhibited significant differences in thermolysin digestion midpoints. Among the group 3 mutants, most possessed significantly different protein stabilities relative to their parental toxins. Two of the group 3 mutants were observed to have exchanged the thermolysin sensitivity properties of the parental toxins.

Biotinylation of Bacillus thuringiensis Insecticidal Crystal Proteins

Biotinylation of Bacillus thuringiensis insecticidal crystal proteins (ICPs) was evaluated for its potential use in an alternative ICP screening method and in the characterization of ICP receptors. In vivo biological activity of CryIA(b), as inferred from bioassays with Manduca sexta and Ostrinia nubilalis and from histopathological effects on O. nubilalis midgut cells induced by force feeding, was not affected by biotinylation at moderate biotinylation ratios. A competitive radioreceptor assay showed that there was only a minor reduction in binding affinity of biotin-labeled CryIA(b) for M. sexta brush border membrane vesicles. On midgut tissue sections, the binding pattern along the midgut epithelium and the staining intensity of biotinylated ICPs detected with streptavidin-enzyme conjugate were virtually identical to the binding pattern and staining intensity of native CryIA(b) detected with antibodies. The specificity of biotinylated ICP binding to larval midgut tissue was demonstrated by performing homologous competition experiments. The relationship between different ICP receptor types in Plutella xylostella, as inferred from radioligand binding studies, was confirmed by the results of heterologous competition experiments performed with biotinylated and native ICPs.

Screening by polymerase chain reaction of Bacillus thuringiensis serotypes for the presence of cryV-like insecticidal protein genes and characterization of a cryV gene cloned from B. thuringiensis subsp. kurstaki

Polymerase chain reaction screening using cryV-specific oligonucleotides, designed to amplify the 5' half of cryV-type genes, revealed the presence of such genes in 7 of 21 Bacillus thuringiensis serotypes examined. Restriction analysis and hybridization studies indicated that these putative genes fall into at least three subclasses. The nucleotide sequence of the cryV-type gene cloned from B. thuringiensis subsp. kurstaki DSIR732 revealed an open reading frame coding for a protein of 719 amino acids, and lysates of Escherichia coli cells expressing the 81.2-kDa CryV732 protein were toxic to Epiphyas postvittana (Lepidoptera: Tortricidae).

Cloning of a novel cryIC-type gene from a strain of Bacillus thuringiensis subsp. galleriae

A novel cryIC-type gene was isolated from a strain of Bacillus thuringiensis subsp. galleriae. A new polymerase chain reaction (PCR) technique with a set of several oligonucleotide primer pairs specific to the cryIC gene was used to screen a number of B. thuringiensis strains. PCR amplified several DNA fragments ranging from 100 bp to 1 kb for B. thuringiensis strains containing a cryIC gene. PCR fragments amplified from the Bacillus thuringiensis subsp. galleriae HD29 DNA differed from the fragments amplified from other cryIC-containing strains, indicating strain HD29 contained a novel cryIC-type gene. To isolate crystal genes homologous to cryIC, an HD29 gene library was probed with a 984-bp fragment of the amino-terminal coding region of the cryIC gene cloned from Bacillus thuringiensis subsp. aizawai HD229. A putative toxin gene was isolated from a phage that hybridized strongly to the cryIC probe. Translation of the putative toxin DNA sequence revealed an open reading frame of 1,176 amino acids whose predicted molecular mass was 132.8 kDa. Comparisons of the toxin gene sequence with sequences of other cry genes indicated that this gene is a subclass of cryIC. We propose to designate this gene cryIC(b). In Escherichia coli, the cryIC(b) gene produced a protein of approximately 130 kDa toxic to Spodoptera exigua and Trichoplusia ni.

The two faces of Bacillus thuringiensis: insecticidal proteins and post-exponential survival

Post-exponential Bacillus thuringiensis cells produce both an endospore and a variety of intracellular inclusions. The latter are comprised of protoxins, each being specific for the larvae of certain species from at least three orders of insects. Following ingestion of spores and inclusions, toxicity results in the spores gaining access to haemolymph, a source of nutrients suitable for germination and growth. Most B. thuringiensis subspecies contain multiple, plasmid-encoded protoxin genes, often with several on the same plasmid. These genes have been manipulated in order to understand the basis of toxicity and specificity, information which is important to the use of these toxins as biological control agents. Some protoxin genes are in operons, and others are in close proximity, perhaps to enhance the chances of recombination, and some are on unstable plasmids. The arrangement of these genes is probably important for flexibility in the variety of protoxins packaged into inclusions by a particular subspecies and thus the capacity to adapt to changing populations of insects. Protoxins accumulate over a prolonged period during sporulation because of the sequential transcription from two promoters, each being dependent upon a specific sporulation sigma factor, the relative stability of the messenger RNA, and the synthesis of proteins which stabilize protoxins and perhaps facilitate inclusion assembly. During the post-exponential phase, spore and inclusion formation must be balanced so as to ensure that both are available to contribute to the survival of these bacilli.

Characterization and properties of a novel plasmid vector for Bacillus thuringiensis displaying compatibility with host plasmids

A novel plasmid vector, composed of a 1.7-kb Bacillus thuringiensis (B.t.) replicon, a multiple cloning site, and an erythromycin-resistance marker gene from Bacillus subtilis, was constructed for use in B.t. Unlike other vectors which have been reported to be acceptable for B.t., this new B.t. vector was stably maintained in the absence of Er and did not displace host plasmids, some of which carry crystal protein-encoding genes (cry genes). The compatibility of this B.t. vector with native plasmids is highly desirable when introducing new cry genes into a wild-type B.t. strain. When a cryIIIA gene of B.t. tenebrionis was cloned in this vector and introduced into B.t. kurstaki (kur) HD119, cryIIIA was highly expressed without affecting the level of expression of native cry genes. The stability of this vector and its compatibility with native B.t. plasmids were achieved by subcloning only nucleotide sequences required for the vector to replicate in B.t. The origin of replication was first cloned on a 9.6-kb Bg/II fragment from a 75-kb plasmid of B.t. kur HD73 and then localized to a 2.4-kb region within the 9.6-kb fragment. Sequencing of the 2.4-kb region revealed the presence of an open reading frame (ORF), encoding a putative 312-amino acid (aa) protein. The deduced aa sequence of the ORF showed no homology to any published aa sequences. Deletion analysis indicated that the B.t. vector required at least the ORF and up to 300 bp surrounding the ORF, in order to replicate.

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

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

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

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

Insecticidal properties of a crystal protein gene product isolated from Bacillus thuringiensis subsp. kenyae

A protoxin gene, localized to a high-molecular-weight plasmid from Bacillus thuringiensis subsp. kenyae, was cloned on a 19-kb BamHI DNA fragment into Escherichia coli. Characterization of the gene revealed it to be a member of the CryIE toxin subclass which has been reported to be as toxic as the CryIC subclass to larvae from Spodoptera exigua in assays with crude E. coli extracts. To directly test the purified recombinant gene product, the gene was subcloned as a 4.8-kb fragment into an expression vector resulting in the overexpression of a 134-kDa protein in the form of phase-bright inclusions in E. coli. Treatment of solubilized inclusion bodies with either trypsin or gut juice from the silkworm Bombyx mori resulted in the appearance of a protease-resistant 65-kDa protein. In force-feeding bioassays, the purified activated protein was highly toxic to larvae of B. mori but not to larvae of Choristoneura fumiferana. In diet bioassays with larvae from S. exigua, the purified protoxin was nontoxic. However, prior activation of the protoxin by tryptic digestion resulted in the appearance of some toxic activity. These results demonstrate that this new subclass of protein toxin may not be useful for the control of Spodoptera species as previously reported. Hierarchical clustering of the nine known lepidopteran-specific CryI toxin subclasses through multiple sequence alignment suggests that the toxins fall into four possible subgroups or clusters.

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

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

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.