Identification of Aedes aegypti specificity motifs in the N-terminus of the Bacillus thuringiensis Cry2Aa pesticidal protein

Characterization of the individual domains of the Bacillus thuringiensis Cry2Aa implicates Domain I as a possible binding site to Helicoverpa armigera

Bacillus thuringiensis (Bt) Cry2Aa is a member of the Cry pore-forming, 3-domain, toxin family with activity against both lepidopteran and dipteran insects. Although domains II and III of the Cry toxins are believed to represent the primary specificity determinant through specific binding to cell receptors, it has been proposed that the pore-forming domain I of Cry2Aa also has such a role. Thus, a greater understanding of the functions of Cry2Aa's different domains could potentially be helpful in the rational design of improved toxins. In this work, cry2Aa and its domain fragments (DI, DII, DIII, DI-II and DII-DIII) were subcloned into the vector pGEX-6P-1 and expressed in Escherichia coli. Each protein was recognized by anti-Cry2Aa antibodies and, except for the DII fragment, could block binding of the antibody to Cry2Aa. Cry2Aa and its DI and DI-II fragments bound to brush border membrane vesicles (BBMV) from H. armigera and also to a ca 150 kDa BBMV protein on a far western (ligand) blot. In contrast the DII, DIII and DII-III fragments bound to neither of these. None of the fragments were stable in H. armigera gut juice nor showed any toxicity towards this insect. Our results indicate that contrary to the general model of Cry toxin activity domain I plays a role in the binding of the toxin to the insect midgut.

N-terminal proteolysis determines the differential activity of Bacillus thuringiensis Cry2A toxins towards Aedes aegypti

It has long been known that while both the Bacillus thuringiensis pesticidal proteins Cry2Aa and Cry2Ab have wide-ranging activities against lepidopteran insects only the former has activity against the mosquito Aedes aegypti. We have previously shown that this differential specificity is influenced by the N-terminal region of these proteins and here demonstrate that this is due to these sections affecting proteolytic activation. Enzymes from the midgut of A. aegypti cleave Cry2Aa at the C-terminal side of amino acid 49 resulting in a 58 kDa fragment whereas these enzymes do not cleave Cry2Ab at this position. The 58 kDa, but not the protoxin, form of Cry2Aa is capable of interacting with brush border membrane vesicles from A. aegypti.

Molecular cloning of a new cry2A-type gene from Bacillus thuringiensis strain Nn10 and its expression studies

In the present study, eight indigenous Bacillus thuringiensis isolates of Western Ghats of India with more than 90% toxicity against Helicoverpa armigera were characterized for cry2A gene sub families. Seven of the eight isolates harboured cry2Aa, cry2Ab and cry2Ac genes alone and or in combination. Further, the indigenous cry2Aa gene(s) from Bacillus thuringiensis isolate Nn10 which showed 100% mortality against Helicoverpa armigera was cloned and expressed into recombinant Bt strains for management of resistance development in insects. The ORF of cry2Aa (∼1.9 kb) gene(s) from Nn10 isolate was ligated with T/A vector (pTZ57 R/T) and expressed in E. coli, DH5α. Automated sequence analysis of newly cloned recombinant cry2Aa revealed 99% homology to 916 bases in the 3' region of minus strand and 100% homology with 720 bases in the 5' region of holotype cry2Aa1 gene. The partial Cry2Aa amino acid sequence of Bt strain, Nn10, deduced from the nucleotide sequence generated by M13F primer showed four amino acid variation in comparison to Cry2Aa1 holotype, at 338, 345, 346 and 489^th position of ORF and the sequence was submitted to the NCBI. Further the expression of ORF of cry2Aa of Nn10 into acrystalliferous Bt strain, 4Q7 using expression vector pHT3P2T under the transcriptional control of cry3Aa promoter and cry2Aa terminator. SDS PAGE analysis of recombinant protein exhibited a prominent band of about 65 kDa. Bioassay studies revealed that recombinant proteins, Cry2Aa of Nn10 was toxic to Helicoverpa armigera with LC50 value of 7.26 μg ml^-1.

Exploitation of Novel Bt ICPs for the Management of Helicoverpa armigera (Hübner) in Cotton (Gossypium hirsutum L.): A Transgenic Approach

Cotton is a commercial crop of global importance. The major threat challenging the productivity in cotton has been the lepidopteron insect pest Helicoverpa armigera or cotton bollworm which voraciously feeds on various plant parts. Biotechnological interventions to manage this herbivore have been a universally inevitable option. The advent of plant genetic engineering and exploitation of Bacillus thuringiensis (Bt) insecticidal crystal proteins (ICPs) marked the beginning of plant protection in cotton through transgenic technology. Despite phenomenal success and widespread acceptance, the fear of resistance development in insects has been a perennial concern. To address this issue, alternate strategies like introgression of a combination of cry protein genes and protein-engineered chimeric toxin genes came into practice. The utility of chimeric toxins produced by domain swapping, rearrangement of domains, and other strategies aid in toxins emerging with broad spectrum efficacy that facilitate the avoidance of resistance in insects toward cry toxins. The present study demonstrates the utility of two Bt ICPs, cry1AcF (produced by domain swapping) and cry2Aa (produced by codon modification) in transgenic cotton for the mitigation of H. armigera. Transgenics were developed in cotton cv. Pusa 8–6 by the exploitation of an apical meristem-targeted in planta transformation protocol. Stringent trait efficacy-based selective screening of T₁ and T₂ generation transgenic plants enabled the identification of plants resistant to H. armigera upon deliberate challenging. Evaluation of shortlisted events in T₃ generation identified a total of nine superior transgenic events with both the genes (six with cry1AcF and three with cry2Aa). The transgenic plants depicted 80–100% larval mortality of H. armigera and 10–30% leaf damage. Molecular characterization of the shortlisted transgenics demonstrated stable integration, inheritance and expression of transgenes. The study is the first of its kind to utilise a non-tissue culture-based transformation strategy for the development of stable transgenics in cotton harbouring two novel genes, cry1AcF and cry2Aa for insect resistance. The identified transgenic events can be potential options toward the exploitation of unique cry genes for the management of the polyphagous insect pest H. armigera.

Potential for Bacillus thuringiensis and Other Bacterial Toxins as Biological Control Agents to Combat Dipteran Pests of Medical and Agronomic Importance

The control of dipteran pests is highly relevant to humans due to their involvement in the transmission of serious diseases including malaria, dengue fever, Chikungunya, yellow fever, zika, and filariasis; as well as their agronomic impact on numerous crops. Many bacteria are able to produce proteins that are active against insect species. These bacteria include Bacillus thuringiensis, the most widely-studied pesticidal bacterium, which synthesizes proteins that accumulate in crystals with insecticidal properties and which has been widely used in the biological control of insects from different orders, including Lepidoptera, Coleoptera, and Diptera. In this review, we summarize all the bacterial proteins, from B. thuringiensis and other entomopathogenic bacteria, which have described insecticidal activity against dipteran pests, including species of medical and agronomic importance.