Identification of the minimal active fragment of the Cry1Ah toxin

Safety of Bacillus thuringiensis Cry1Ah and Vip3Aa toxins for the predatory stink bug Arma custos (Hemiptera: Pentatomidae)

The widespread adoption of Bt crops expressing insecticidal proteins derived from Bacillus thuringiensis has created a need to assess the potential effects of these toxins on non-target organisms, especially species such as Arma custos, a generalist predator that provides important biological control services in many field crops in Asia. Direct dietary exposure of A. custos to Cry1Ah and Vip3Aa proteins produced no adverse effects on life history traits, despite continuous exposure throughout development and early adult life to concentrations significantly higher than the Bt protein concentration likely encountered by A.custos in the field, even when feeding directly on Bt plants. Enzyme-linked immunosorbent assay confirmed the presence of Bt proteins in A. custos midguts, but quantitative real-time PCR analysis of 12 genes associated with detoxification, antioxidative responses, immune responses, and metabolism revealed no significant changes in expression in adult bugs. Indirect exposure to these toxins via consumption of intoxicated prey, larvae of Helicoverpa armigera (Hübner), likewise produced no negative impacts on survival, development, adult weight, or female fecundity in either the F0 (exposed) or F1 (unexposed) generation, but female fresh weight was reduced in the F0 generation by the Cry1Ah (50 μg/g) treatment. Finally, a competitive binding assay with labelled protein and a ligand blotting assay both demonstrated that the Cry1Ah protein could not bind to receptors on the midgut brush border membrane vesicles (BBMVs) of A. custos adults. Therefore, we conclude that Cry1Ah and Vip3Aa proteins are unlikely to have significant negative effects on A. custos populations if employed as plant-incorporated protectants in field crops.

Insights from the Structure of an Active Form of Bacillus thuringiensis Cry5B

The crystal protein Cry5B, a pore-forming protein produced by the soil bacterium Bacillus thuringiensis, has been demonstrated to have excellent anthelmintic activity. While a previous structure of the three-domain core region of Cry5B(112-698) had been reported, this structure lacked a key N-terminal extension critical to function. Here we report the structure of Cry5B(27-698) containing this N-terminal extension. This new structure adopts a distinct quaternary structure compared to the previous Cry5B(112-698) structure, and also exhibits a change in the conformation of residues 112-140 involved in linking the N-terminal extension to the three-domain core by forming a random coil and an extended α-helix. A role for the N-terminal extension is suggested based on a computational model of the tetramer with the conformation of residues 112-140 in its alternate α-helix conformation. Finally, based on the Cry5B(27-698) structure, site-directed mutagenesis studies were performed on Tyr495, which revealed that having an aromatic group or bulky group at this residue 495 is important for Cry5B toxicity.

Characterization of a new cry2Ab gene of Bacillus thuringiensis with high insecticidal activity against Plutella xylostella L

Bacillus thuringiensis (Bt) strain FJAT-12 was a novel Bt strain isolated by Agricultural Bio-Resources Institute, Fujian Academy of Agricultural Science. In this study, a new cry2Ab gene was cloned from Bt strain FJAT-12 and named as cry2Ab30 by Bt delta-endotoxin Nomenclature Committee. The sequencing results showed there were two mutations in conservative sites which led to two amino acids modification. Homology modeling indicated that the two changes were located in β-sheet of Domain II. A prokaryotic expression vector pET30a-cry2Ab30 was constructed and the expressed protein was analyzed by western blot using Cry2Ab antibody. The expression conditions including IPTG concentration, revolution and temperature were optimized to get the highest expression level by SDS-PAGE and BandScan. The bioassay results also showed that the Cry2Ab30 toxin had high insecticidal activity against Plutella xylostella and the LC50 value was 0.0103 μg.mL(-1). The two mutations in β-sheet of Domain II might contribute to insecticidal activity of Cry2Ab30 toxin against Plutella xylostella.