Complete Genome Sequence of Bacillus thuringiensis Mutant Strain BMB171

Bacillus thuringiensis has been widely used as a biopesticide for a long time. Here we report the finished and annotated genome sequence of B. thuringiensis mutant strain BMB171, an acrystalliferous mutant strain with a high transformation frequency obtained and stocked in our laboratory.Bacillus thuringiensis is an insect pathogen which is widely used as a biopesticide due to its various endogenous crystal proteins and spores (12). To improve the virulence and practical effectiveness of B. thuringiensis, genetic transformation of different genes with beneficial traits is a fundamental procedure. Simultaneously, genetic transformation can facilitate functional genomic research. However, wild-type strains are not suitable to be used as recipient strains because of low transformation efficiency. This obstacle is mainly caused by the thick cell wall layer of B. thuringiensis together with multiple plasmids inside the cell, which harbor genes encoding insecticidal crystal proteins. We used the method of elevating the growth temperature and adding 0.05% sodium dodecyl sulfate to treat several parental strains and finally obtained mutant strain BMB171, with no resident plasmid, from wild-type crystalliferous strain YBT-1463 (9). The electrotransformation frequency of mutant BMB171 could reach up to 10⁷ transformants/μg DNA after optimization of the electrotransformation parameters (7), which was 4.8 × 10⁴-fold higher than that of the parental strain (8). Moreover, mutant strain BMB171 exhibited the same characteristics as YBT-1463, such as metabolic abilities and growth properties, as well as sensitivity to 10 antibiotics (8). Of course, BMB171 could produce parasporal crystals with characteristic geometric shapes through the expression of relevant cry genes carried by plasmids (7). Thus, B. thuringiensis mutant strain BMB171 has become a major recipient strain and is widely used for insecticidal crystal protein-encoding gene expression (14, 15), cell surface display (10, 13), gene function and regulation researches (2, 5), etc.The B. thuringiensis mutant strain BMB171 genome was sequenced by using a massive parallel pyrosequencing technology (454 GS-FLX). A total of 448,963 high-quality reads with an average read length of 391 bp were produced, providing about 32-fold coverage of the genome. Assembly was performed using the Newbler software of the 454 suite package (454 Life Sciences), which resulted in 193 large (defined as >500 bp) contigs. The relationship of contigs was determined by multiplex PCR, and gaps were filled through sequencing of PCR products by primer walking or shotgun sequencing with an ABI 3730 sequencer. The Phred/Phrap/Consed software package (3) was used for final sequence assembly and quality assessment. Protein-coding genes were predicted by combining the results of Glimmer 3.02 (1) and ZCURVE (4), followed by manual inspection. Both tRNA and rRNA genes were identified by tRNAscan-SE (11) and RNAmmer (6), respectively. Functional annotation was performed by searching against a protein database of the microbial genome developed in house.The 5.64-Mb genome of B. thuringiensis mutant strain BMB171 contains two replicons: a circular chromosome (5.33 Mb) encoding 5,088 open reading frames (ORFs) and a circular plasmid (0.31 Mb), which is named pBMB171, encoding 276 predicted ORFs. The G+C content of the chromosome is 35.3%, while that of the plasmid is 33.3%. The mutant strain BMB171 genome encodes 104 tRNAs and 14 rRNA operons. A previous study indicated that BMB171 is a plasmid-free mutant (9); however, our sequencing results demonstrated that a large plasmid still remains. The reason why the plasmid was not detected previously might be its large size and low copy number. We did not find any crystal protein genes in either chromosome or plasmid sequences, which was consistent with previous observations (9).In summary, the complete B. thuringiensis mutant strain BMB171 genome provides a better-defined genetic background for gene expression and regulation studies, especially crystal protein production and metabolic network construction.     

The cry1Ac gene of Bacillus thuringiensis ZQ-89 encodes a toxin against long-horned beetle adult

Aims: Bacillus thuringiensis is toxic to many insects including Coleopteran pests. However, there is no report that B. thuringiensis is toxic to the adults of long‐horned beetle, Batocera horsfieldi, a pest of poplar trees. This work aims to select a B. thuringiensis strain toxic towards the adults of Asian long‐horned beetle. Methods and Results: A total of 504 B. thuringiensis strains were tested for the insecticidal activity to B. horsfieldi adults by artificial feeding. A strain of ZQ‐89 was found with a high toxicity to B. horsfieldi adults. The rectified lethal rate of ZQ‐89 to beetle was 55·33%. Additionally, the body weight and egg‐hatching rate of beetle, respectively, decreased by 2·22 and 19·62% after being fed with ZQ‐89. Further investigation found that the pure parasporal crystal had high toxicity to beetle adults. The ZQ‐89 crystal protein was purified and analysed by peptide‐mapping fingerprint and found it was highly homologous to Cry1Ac protein. The crystal protein gene was cloned and named cry1Ac89. The cry1Ac89 gene and its promoter were inserted into the plasmid pHT304 and then transformed into B. thuringiensis acrystalliferous strain BMB171. SDS‐PAGE analysis showed the BMB171‐Cry1Ac89 recombinant strain successfully expresses a 133‐kDa recombinant crystal protein with highly lethal activity to B. horsfieldi adults. Conclusions: The strain of ZQ‐89 is highly toxic to the adults of long‐horned beetle, and the crystal protein mainly contributes to the antipest role of this strain. The cry1Ac89 gene is a good candidate to be used for making transgenic trees or develop environment‐friendly bioinsecticides against long‐horned beetles such as B. horsfieldi in the future. Significance and Impact of the Study: It is the first report of a B. thuringiensis strain toxic to the adults of Asian long‐horned beetle, and the Cry1Ac protein is also firstly reported to be toxic to Coleopteran pests.     

Diversity analysis and characterization of Coleoptera-, Hemiptera- and Nematode-active cry genes in native isolates of Bacillus thuringiensis

Applications to combat non-lepidopteran insects are not as common as applications against lepidopteran insects. The aim of the present work was to isolate and identify Bacillus thuringiensis isolates from soil samples using five approaches, viz., analysis of crystal protein production by microscopy; detection of cry gene content by PCR, SDS-PAGE profiling; cloning and sequencing; phylogenetic analysis; and toxicity testing. Two hundred soil samples were used for isolation of B. thuringiensis and a total of 69 putative isolates of B. thuringiensis that produce parasporal crystalline inclusions were isolated from 5,267 Bacillus-like colonies. A bipyramidal inclusion was predominant in 32.2 % of the B. thuringiensis isolates compared to other shapes. Crystal protein profiling of B. thuringiensis isolates by SDS-PAGE analysis showed the presence of bands of 130, 73, 34, 25 and 13 kDa, among which 50–60 kDa bands were present abundantly. PCR analysis revealed the predominance of Coleopteran-active cry genes in these isolates. Variation in nucleotide sequences, crystal morphology and mass of crystal protein(s) purified from the isolates of B. thuringiensis revealed genetic and molecular diversity. Four strains containing Coleopteran-active cry genes showed higher toxicity against Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae) adults when compared with B. thuringiensis subsp. morrisoni pathovar tenebrionis. These results are useful in emphasizing the distribution of cry genes and for prognostication of toxicity, and may contribute to the identification of novel candidate genes for bioengineered crop protection.     

苏云金芽胞杆菌幕虫亚种晶胞粘连现象与晶体蛋白基因cry26Aa所在质粒有关

苏云金芽胞杆菌幕虫亚种T02菌株的伴胞晶体在芽胞外壁内侧形成,呈现晶胞粘连的现象。在此菌株中克隆了cry26Aa和cry28Aa两个基因,并对晶胞粘连现象与质粒的相关性做了系统研究。通过消除幕虫亚种T02菌株的质粒,得到了仅消除cry26Aa所在质粒的菌株BMB1151和无质粒的菌株BMB1152。通过穿梭载体将cry26Aa和cry28Aa两个基因分别和同时转化无质粒突变株BMB1152并表达,形成的晶体与芽胞独立存在不能粘连,表明在幕虫亚种染色体背景下仅仅cry的表达不能形成晶胞粘连现象,从而推断晶胞粘连现象可能与幕虫亚种两个基因所在的质粒有关;进一步的研究发现将cry26Aa在仅消除cry26Aa所在质粒的突变株BMB1151中表达,形成的晶体与芽胞也分别独立存在不能粘连,从而进一步推断幕虫亚种晶胞粘连现象与cry26Aa所在质粒有关。     

Characterisation of Bacillus thuringiensis mutant highly producing melanin pigment and active against potato tuber moth

The bacteria Bacillus thuringiensis mutant is highly producing melanin pigment with increased ultra violet resistance and insecticidal activity against the potato tuber moth Phthorimaea operculella (Zeller). The results showed that the high decrease of crystal protein formation rate ranged from 100% (B.t.EMS-M2 and B.t.EMS-M6) to 91.82% (B.t.EMS-M9). The EMS–UV-induced mutants (B.t.EMS–UV-2h-1, B.t. EMS–UV-2h-2, B.t.EMS–UV-2h-3, B.t.EMS–UV-2h-5, B.t.EMS–UV-4h-1, B.t.EMS–UV-4h-3 and B.t.EMS–UV-6h-2) showed 100% decrease in the crystal protein formation. Results also showed that the growth rate of B. thuringiensis isolates was detected by measuring the light absorption of culture broth (BP media at pH 8) at the wavelength of 600 nm. The absorbance values of the standard melanin were 2.055 and 0.134 at wavelengths of 226.5 and 602 nm, respectively. This means that the maximum absorbance at wavelength was 226.5 nm, this result is similar to that of the synthetic melanin which has the absorbance of 226 nm. Our experiments detected that the pigment extracted from the mutant isolate B.t.EMS-M3 (EMS-induced mutant) gave the maximum value of absorbance (2.615) at wavelength of 227.5 nm that was similar to standard melanin which gave absorbance value about 2.055 at a wavelength of 226.5 nm. This may be due to the genetic alterations that happened to the mutant isolates due to the mutation by EMS or/and UV irradiation.     

In vitro hydrolysis of Bacillus thuringiensis Cry1Ac toxin by gut proteases of Nilaparvata lugens (Stål) and binding assays of Cry1Ac toxin with brush border membrane of N. lugens midgut

Bacillus thuringiensis (Bt) and transgenic crops carrying cry genes are widely used in the management of lepidopteran and coleopteran pests. However, almost none of the Cry toxins have insecticidal properties against sap-sucking insects, such as planthoppers, leafhoppers and aphids. To understand the low insecticidal activity of Cry1Ac toxin on sap-sucking insects, we investigated two critical steps in the Bt-intoxication cascade: the proteolytic processing of Cry1Ac toxin by gut proteases, and the binding of Cry1Ac to brush border membrane vesicles (BBMV) of Nilaparvata lugens. Proteolytic processing of Cry1Ac protoxin by N. lugens gut proteases resulted in an ～65?kDa product, similar to the expected size of the trypsin-activated Cry1Ac toxin. In addition, activation of cysteine proteases in N. lugens gut increased the efficiency of proteolytic activities in the processing of Cry1Ac. However, feeding N. lugens nymphs with either Cry1Ac protoxin or trypsin-activated Cry1Ac toxin resulted in low mortalities. The LC₅₀ of Cry1Ac protoxin and trypsin-activated Cry1Ac was 198.92 and 450.18?μg/mL, respectively. In vitro binding analysis of BBMV with the pre-activated Cry1Ac showed that Cry1Ac toxin could specifically bind to the BBMV. However, binding competition with 500-fold molar excess GalNAc (N-acetyl-d-galactosamine) suggested that the binding was not mediated by GalNAc-like glycoproteins. These results indicate that Cry1Ac toxin could be successfully processed by the treatment of N. lugens gut proteases. However, the binding of Cry1Ac toxin to the midgut brush border membrane was not mediated by GalNAc-like glycoprotein. This may be responsible for the low susceptibility of N. lugens to Cry1Ac.     

Expression of <Emphasis Type="Italic">Vitreoscilla</Emphasis> hemoglobin in <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> improve the cell density and insecticidal crystal proteins yield

The Vitreoscilla hemoglobin (VHb) gene (vgb) was integrated into the chromosome of Bacillus thuringiensis BMB171 using integrative vector pEG491. The production of VHb was confirmed by CO-difference spectra analysis. Fermentation experiments results showed that with the production of VHb, the critical oxygen concentration (COC) of the host strain was reduced from 18 to 12%. The maximum viable cell counts of the VHb⁺ strain in high, middle, and low aeration/agitation fermentations were 0.94-, 1.23-, and 1.59-fold of those of the VHb⁻ strain, respectively. Under the same conditions, the yields of insecticidal crystal proteins (ICP) by VHb⁺ strain were 1.22-, 1.63-, and 3.13-fold of those of the VHb⁻ strain. The production of VHb also accelerated the formation of ICP and spores. These results indicated that the production of VHb could improve the cell density and ICP yield of B. thuringiensis, especially under low aeration/agitation condition.     

Genetically modified coffee plants expressing the Bacillus thuringiensis cry1Ac gene for resistance to leaf miner

 A synthetic version of the cry1Ac gene of Bacillus thuringiensis has been used for the transformation of coffee species (Coffea canephora and C. arabica) to confer resistance to an important pest, the coffee leaf miner (Perileucoptera coffeella and other Leucoptera spp). Somatic embryos were co-cultivated with the LBA4404 strain of Agrobacterium tumefaciens containing the cry1Ac gene. More than 100 transformed plants from independent transformation events were obtained for each coffee genotype. The integration and expression of the cry1Ac gene was studied, and effective resistance of transgenic plants against leaf miner was verified in bioassays with the insects. These plants could represent a good opportunity to analyse the impact of genetic engineering of perennial crops for sustainable resistance to an obligate endocarpic pest using a B. thuringiensis insecticidal protein. Received: 7April 1999 / Revision received: 20 July 1999 / Accepted: 22 July 1999     

Characterization of melanin produced by a wild-type strain of Bacillus cereus

Bacillus cereus 58 (Bc58) is a UV-resistant wild type strain that has an ability to produce a sorrel pigment induced by L-tyrosine. The Fourier-transform infrared (FT-IR) spectra and chemical tests of its pigment are similar to that of the standard melanin (Sigma). A bioassay shows that the LC₅₀ of a Bacillus thuringiensis (Bt) formulation added with the melanin of Bc58 and exposed to UV for 5 h is 16.1 μg/ml, which is similar to that of the Bt formulation without UV treatment, however, it is almost double that of the Bt formulation exposed to UV without the melanin of Bc58. The result of SDS-PAGE indicates that the melanin of Bc58 can protect the insecticidal crystal proteins from degradation. This suggests that it is an excellent UV protective agent for the insecticidal crystal proteins of the Bt formulation. Translated from Microbiology, 2006, 33(1): 42–45 [译自: 微生物学通报]     

Molecular Characterization and Genetic Diversity of Insecticidal Crystal Protein Genes in Native Bacillus thuringiensis Isolates

The Western Ghats of Karnataka natural ecosystem are among the most diverse and is one of the eight hottest hotspots of biological diversity in the world, that runs along the western part of India through four states including Karnataka. Bacillus thuringiensis (Bt) strains were isolated from soils of Western Ghats of Karnataka and characterized by molecular and analytical methods as a result of which 28 new Bt-like isolates were identified. Bt strains were isolated from soil samples using sodium acetate selection method. The morphology of crystals was studied using light and phase contrast microscopy. Isolates were further characterized for insecticidal cry gene by PCR, composition of toxins in bacterial crystals by SDS-PAGE cloning, sequencing and evaluation of toxicity was done. As a result 28 new Bt-like isolates were identified. Majority of the isolates showed the presence of a 55 kDa protein bands on SDS-PAGE while the rest showed 130, 73, 34, and 25 kDa bands. PCR analysis revealed predominance of Coleopteran-active cry genes in these isolates. The variations in the nucleotide sequences, crystal morphology, and mass of crystal protein(s) purified from the Bt isolates revealed genetic and molecular diversity. Three strains containing Coleopteran-active cry genes showed higher activity against larvae Myllocerus undecimpustulatus undatus Marshall (Coleoptera: Curculionidae) than B. thuringiensis subsp. Morrisoni. Results indicated that Bt isolates could be utilized for bioinsecticide production, aiming to reduce the use of chemical insecticide which could be useful to use in integrated pest management to control agriculturally important pests for sustainable crop production.     

Isolation and characterization of a new Bacillus thuringiensis strain Lip harboring a new cry1Aa gene highly toxic to Ephestia kuehniella (Lepidoptera: Pyralidae) larvae

The aim of this study was to characterize new Bacillus thuringiensis strains that have a potent insecticidal activity against Ephestia kuehniella larvae. Strains harboring cry1A genes were tested for their toxicity, and the Lip strain showed a higher insecticidal activity compared to that of the reference strain HD1 (LC₅₀ of Lip and HD1 were 33.27 and 128.61 μg toxin/g semolina, respectively). B. thuringiensis Lip harbors and expresses cry1Aa, cry1Ab, cry1Ac, cry1Ad and cry2A. DNA sequencing revealed several polymorphisms in Lip Cry1Aa and Cry1Ac compared to the corresponding proteins of HD1. The activation process using Ephestia kuehniella midgut juice showed that Lip Cry1A proteins were more stable in the presence of larval proteases. Moreover, LipCry1A proteins exhibited higher insecticidal activity against these larvae. These results indicate that Lip is an interesting strain that could be used as an alternative to the worldwide used strain HD1.     

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.     

A Cry1Ac Toxin Variant Generated by Directed Evolution has Enhanced Toxicity against Lepidopteran Insects

Cry1Ac insecticidal crystal proteins produced by Bacillus thuringiensis (Bt) have become an important natural biological agent for the control of lepidopteran insects. In this study, a cry1Ac toxin gene from Bacillus thuringiensis 4.0718 was modified by using error-prone PCR, staggered extension process (StEP) shuffling combined with Red/ET homologous recombination to investigate the insecticidal activity of delta-endotoxin Cry1Ac. A Cry1Ac toxin variant (designated as T524N) screened by insect bioassay showed increased insecticidal activity against Spodoptera exigua larvae while its original insecticidal activity against Helicoverpa armigera larvae was still retained. The mutant toxin T524N had one amino acid substitution at position 524 relative to the original Cry1Ac toxin, and it can accumulate within the acrystalliferous strain Cry-B and form more but a little smaller bipyramidal crystals than the original Cry1Ac toxin. Analysis of theoretical molecular models of mutant and original Cry1Ac proteins indicated that the mutation T524N located in the loop linking β16–β17 of domain III in Cry1Ac toxin happens in the fourth conserved block which is an arginine-rich region to form a highly hydrophobic surface involving interaction with receptor molecules. This study showed for the first time that single mutation T524N played an essential role in the insecticidal activity. This finding provides the biological evidence of the structural function of domain III in insecticidal activity of the Cry1Ac toxin, which probably leads to a deep understanding between the interaction of toxic proteins and receptor macromolecules.     

苏云金芽胞杆菌营养期杀虫蛋白Vip与Cry蛋白的协同作用

将构建的营养期杀虫蛋白基因vip3表达质粒pBMB2328和含杀虫晶体蛋白基因(crylAc10或crylCa)质粒同时电转化无质粒突变株BMB171并双抗筛选。经PCR特异引物扩增验证,分别得到含crylAc10和vip83、crylCa和vip83的双基因重组菌BMB2830-171和BMB2882-171。用单基因重组菌作对照,分别测定了营养期杀虫蛋白Vip83与杀虫晶体蛋白CrylAc10和Cry1Ca两组蛋白对3种重要鳞翅目害虫毒力。经统计分析,结果表明两组杀虫蛋白Vip83与CrylAc10和Vip83与CrylCa之间对棉铃虫均存在拮抗作用,对甜菜夜蛾协同作用不明显;但对小菜蛾前协同作用不明显,而后则有增效作用,其共毒系数为32．6。双基因的遗传稳定性检测表明这种正负协同关系具有一定的分子遗传稳定性,可为高效广谱工程菌的构建提供依据。     

Molecular and biological characterization of native <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> strains for controlling tomato leafminer (<Emphasis Type="Italic">Tuta absoluta</Emphasis> Meyrick) (Lepidoptera: Gelechiidae) in Colombia

Twenty-eight soil samples were obtained from open fields and greenhouses used for tomato cultivation in various regions of Colombia. For functional characterization, 99 Bacillus thuringiensis (Bt) strains were isolated and characterized by abundance and morphology of microscopic crystals, SDS–PAGE of protein extracts and M-PCR analyses of genes of the cry1 family, as well as for their insecticidal activity against Tuta absoluta second instar larvae. Native Bt strains had amorphous (5%), bi-pyramidal (27%), square (8%), spherical (38%) and triangular (22%) crystal forms. Based on the presence of 1–4 different crystal forms, 18 different profiles were established. The SDS–PAGE analyses of protein extracts established ten different strain groups based on their protein band weight and potential biological activity. The M-PCR technique identified 35 native Bt strains based on the presence of the 6 genes cry1Aa, cry1Ab, cry1Ac, cry1B, cry1C and cry1D, whose frequency of occurrence was 76, 26, 21, 35, 32 and 8.8%, respectively. Thirteen different PCR profiles were found in native Bt strains. Several gene combinations tended to co-occur with elevated frequency, such as the pairs cry1Ac/cry1C, cry1Ab/cry1Ac and cry1Ab/cry1B, for which Pearson correlation coefficients were 0.69, 0.52 and 0.54, respectively. Native strains ZBUJTL39 and ZCUJTL11 had up to three times higher biological activity against T. absoluta second instar larvae than the reference strain Bt var. kurstaki HD1, with an LD₅₀ of 2.4 μg/ml (P < 0.05) for native Bt strain ZCUJTL11. This study suggests a high biodiversity of native Bt strains from tomato growing regions in Colombia, which has important implications for designing biological control strategies for T. absoluta.     

Bacillus thuringiensis Bel Protein Enhances the Toxicity of Cry1Ac Protein to Helicoverpa armigera Larvae by Degrading Insect Intestinal Mucin

Bacillus thuringiensis has been used as a bioinsecticide to control agricultural insects. Bacillus cereus group genomes were found to have a Bacillus enhancin-like (bel) gene, encoding a peptide with 20 to 30% identity to viral enhancin protein, which can enhance viral infection by degradation of the peritrophic matrix (PM) of the insect midgut. In this study, the bel gene was found to have an activity similar to that of the viral enhancin gene. A bel knockout mutant was constructed by using a plasmid-free B. thuringiensis derivative, BMB171. The 50% lethal concentrations of this mutant plus the cry1Ac insecticidal protein gene were about 5.8-fold higher than those of the BMB171 strain. When purified Bel was mixed with the Cry1Ac protein and fed to Helicoverpa armigera larvae, 3 μg/ml Cry1Ac alone induced 34.2% mortality. Meanwhile, the mortality rate rose to 74.4% when the same amount of Cry1Ac was mixed with 0.8 μg/ml of Bel. Microscopic observation showed a significant disruption detected on the midgut PM of H. armigera larvae after they were fed Bel. In vitro degradation assays showed that Bel digested the intestinal mucin (IIM) of Trichoplusia ni and H. armigera larvae to various degrading products, similar to findings for viral enhancin. These results imply Bel toxicity enhancement depends on the destruction of midgut PM and IIM, similar to the case with viral enhancin. This discovery showed that Bel has the potential to enhance insecticidal activity of B. thuringiensis-based biopesticides and transgenic crops.Bacillus thuringiensis is a ubiquitous gram-positive, spore-forming soil bacterium and produces insecticidal crystal proteins during the sporulation phase of its growth cycle. Because these insecticidal crystal proteins have activity against certain insect species, B. thuringiensis has been extensively used as a biopesticide to control crop pests in commercial agriculture and forest management. It is also a key source of genes for transgenic expression and provides pest resistance in plants (2, 20, 30).The viral enhancin protein was originally described for granuloviruses (GVs) as a 126-kDa protein that showed an ability to enhance the infectivity of nucleopolyhedroviruses (NPVs) (36, 37, 39). It has also been found in several other GVs (13) and NPVs (19, 27). Considered a pathogenicity factor, it is not essential for growth of viruses in cell culture or infected insects but has the function of facilitating GV and NPV infection and decreasing larval survival time (14, 17, 19, 27).The widely accepted action mode of the viral enhancin protein, which has been identified as a metalloprotease (17), is that it can disrupt the protective peritrophic matrix (PM), allowing virion access to the underlying epithelial cells of the insect gut (17). The PM has a lattice structure formed by chitin and insect intestinal mucin (IIM), and the viral enhancin protein targets the IIM for degradation (33).Enhancin-like genes with 24 to 25% nucleotide identity to viral enhancin genes have been found in Yersinia pestis, Bacillus anthracis, Bacillus thuringiensis, and Bacillus cereus genome sequences (16, 25, 28). When B. cereus enhancin-like protein was expressed in recombinant Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) budded viruses and polyhedral inclusion bodies, it was found to be cytotoxic compared to viral enhancin protein. However, larval bioassays indicated that this enhancin-like protein did not enhance infection (8). Hajaij-Ellouze et al. (12) isolated a B. thuringiensis enhancin-like gene from a 407 crystal-minus strain and found that this enhancin-like protein has a typical metalloprotease zinc-binding domain (HEIAH) and belongs to the PlcR regulon. When the enhancin-like mutant was fed to Galleria mellonella larvae, no significant reduction in virulence was observed.In the present study, we report a B. thuringiensis enhancin-like gene (bel) encoding a protein (Bel) that has 20 to 30% identity to the viral enhancin protein and 95% identity to bacterial enhancin-like proteins. Therefore, Bel function may have a synergistic action similar to that of the virus enhancin protein. To understand the biochemical activity of this novel bacterial gene, bel was knocked out in the plasmid-free strain BMB171. We expected that this bel mutant would have no significant reduction in toxicity according to the reports of Galloway et al. (8) and Hajaij-Ellouze et al. (12). However, the bel mutant surprisingly resulted in dramatically reduced Cry1Ac toxicity to Helicoverpa armigera larvae. To further confirm this result, purified Bel was fed together with the Cry1Ac protein to H. armigera larvae. We found that Bel can function as a synergist of Cry1Ac toxicity against H. armigera. In vivo and in vitro observations showed that Bel can disrupt the insect midgut PM and degrade IIM of insect midgut PM. The target of Bel is the IIM of PM, similar to the results found with viral enhancin.     

Novel Bacillus thuringiensis Binary Insecticidal Crystal Proteins Active on Western Corn Rootworm, Diabrotica virgifera virgifera LeConte

A new family of insecticidal crystal proteins was discovered by screening sporulated Bacillus thuringiensis cultures for oral activity against western corn rootworm (WCR) larvae. B. thuringiensis isolates PS80JJ1, PS149B1, and PS167H2 have WCR insecticidal activity attributable to parasporal inclusion bodies containing proteins with molecular masses of ca. 14 and 44 kDa. The genes encoding these polypeptides reside in apparent operons, and the 14-kDa protein open reading frame (ORF) precedes the 44-kDa protein ORF. Mutagenesis of either gene in the apparent operons dramatically reduced insecticidal activity of the corresponding recombinant B. thuringiensis strain. Bioassays performed with separately expressed, biochemically purified 14- and 44-kDa polypeptides also demonstrated that both proteins are required for WCR mortality. Sequence comparisons with other known B. thuringiensis insecticidal proteins failed to reveal homology with previously described Cry, Cyt, or Vip proteins. However, there is evidence that the 44-kDa polypeptide and the 41.9- and 51.4-kDa binary dipteran insecticidal proteins from Bacillus sphaericus are evolutionarily related. The 14- and 44-kDa polypeptides from isolates PS80JJ1, PS149B1, and PS167H2 have been designated Cry34Aa1, Cry34Ab1, and Cry34Ac1, respectively, and the 44-kDa polypeptides from these isolates have been designated Cry35Aa1, Cry35Ab1, and Cry35Ac1, respectively.     

Characterization of Chimeric Bacillus thuringiensis Vip3 Toxins

Bacillus thuringiensis vegetative insecticidal proteins (Vip) are potential alternatives for B. thuringiensis endotoxins that are currently utilized in commercial transgenic insect-resistant crops. Screening a large number of B. thuringiensis isolates resulted in the cloning of vip3Ac1. Vip3Ac1 showed high insecticidal activity against the fall armyworm Spodoptera frugiperda and the cotton bollworm Helicoverpa zea but very low activity against the silkworm Bombyx mori. The host specificity of this Vip3 toxin was altered by sequence swapping with a previously identified toxin, Vip3Aa1. While both Vip3Aa1 and Vip3Ac1 showed no detectable toxicity against the European corn borer Ostrinia nubilalis, the chimeric protein Vip3AcAa, consisting of the N-terminal region of Vip3Ac1 and the C-terminal region of Vip3Aa1, became insecticidal to the European corn borer. In addition, the chimeric Vip3AcAa had increased toxicity to the fall armyworm. Furthermore, both Vip3Ac1 and Vip3AcAa are highly insecticidal to a strain of cabbage looper (Trichoplusia ni) that is highly resistant to the B. thuringiensis endotoxin Cry1Ac, thus experimentally showing for the first time the lack of cross-resistance between B. thuringiensis Cry1A proteins and Vip3A toxins. The results in this study demonstrated that vip3Ac1 and its chimeric vip3 genes can be excellent candidates for engineering a new generation of transgenic plants for insect pest control.     

Bacillus thuringiensis subsp. sichuansis strain MC28 produces a novel crystal protein with activity against Culex quinquefasciatus larvae

The Bacillus thuringiensis subsp. sichuansis MC28 strain produces spherical parasporal crystals during sporulation and exhibits remarkable insecticidal activity against dipteran and lepidopteran pests. We characterized a novel cry gene (cry69Aa1), which was found in the pMC95 plasmid of the MC28 strain. The cry69Aa1 gene was inserted into a shuttle vector (pSTK) and expressed in an acrystalliferous mutant B. thuringiensis HD73^?. In this transformant, a large number of spherical parasporal crystals, which were toxic to Culex quinquefasciatus (Diptera), were formed.     

Expression of the full-length and 3'-spliced cry1Ab gene in the 135-kDa crystal protein minus derivative of Bacillus thuringiensis subsp. kyushuensis

Bacillus thuringiensis produces a 130–135-kDa insecticidal protein in the form of bipyramidal crystal which is toxic to lepidopteran larvae. Part of the C-terminal region of the native Cry1Ab was replaced by a heterologous sequence of Cry11Aa C-terminus to get a 3′-spliced cry1Ab gene. The full-length cry1Ab and 3′-spliced cry1Ab, which were both cloned into the E. coli–B. thuringiensis shuttle expression vector pHZB1, were expressed in a 135-kDa crystal protein minus derivative of B. thuringiensis subsp. kyushuensis (4U1-Cry⁻¹³⁵). The crystal shape of Cry1Ab proteins from both recombinants was regularly bipyramidal, while the crystal size of the intact Cry1Ab was approximately fivefold larger than the 3′-spliced Cry1Ab. In addition, these two kinds of Cry1Ab proteins had similar toxicity against Argyrogramma agnata larvae. Received: 19 October 2001 / Accepted: 7 December 2001