Tyrosine-776 of Vip3Aa64 from Bacillus thuringiensis is Important for Retained Larvicidal Activity During High-Temperature Storage.

Vip3Aa (vegetative insecticidal protein) secreted by Bacillus thuringiensis (Bt) is highly toxic to lepidopteran insects. The Bt isolate M190 produces Vip3Aa35 at high concentrations, and Vip3Aa35 was found to be very effective against Spodoptera exigua. Unfortunately, the use of Vip3Aa35 in pest control is limited by its short shelf life when stored at high temperatures, retaining activity for only 1 month at 37 °C. To find a more stable alternative, we screened 500 isolates of Bt collected from various locations in Thailand and discovered Bt isolate 294 which produced large amounts of Vip3Aa64 that exhibited high toxicity against S. exigua but could be stored at 37 °C for up to 3 months. Vip3Aa35 and Vip3Aa64 have only nine amino acid differences between them, with six of those residues being located at the C terminus. Vip3Aa35 and Vip3Aa64 chimeras revealed that the C-terminal sequence is important for the retained larvicidal activity observed with Vip3Aa64. Various single amino acid substitutions were created to identify the key amino acids responsible for this stability. A single residue, Tyr776, was found to be solely responsible, with the Vip3Aa35:N776Y acquiring thermostability similar to Vip3Aa64 while the Vip3Aa64:Y776N exhibited Vip3Aa35-like thermostability.

Isoleucine at position 150 of Cyt2Aa toxin from Bacillus thuringiensis plays an important role during membrane binding and oligomerization.

Cyt2Aa2 is a mosquito larvicidal and cytolytic toxin produced by Bacillus thuringiensis subsp. darmstadiensis. The toxin becomes inactive when isoleucine at position 150 was replaced by alanine. To investigate the functional role of this position, Ile150 was substituted with Leu, Phe, Glu and Lys. All mutant proteins were produced at high level, solubilized in carbonate buffer and yielded protease activated product similar to those of the wild type. Intrinsic fluorescence spectra analysis suggested that these mutants retain similar folding to the wild type. However, mosquito larvicidal and hemolytic activities dramatically decreased for the I150K and were completely abolished for I150A and I150F mutants. Membrane binding and oligomerization assays demonstrated that only I150E and I150L could bind and form oligomers on lipid membrane similar to that of the wild type. Our results suggest that amino acid at position 150 plays an important role during membrane binding and oligomerization of Cyt2Aa2 toxin.

Bacillus sphaericus Mtx1 and Mtx2 toxins co-expressed in Escherichia coli are synergistic against Aedes aegypti larvae.

Mtx1 and Mtx2 are mosquitocidal toxins produced by some strains of Bacillus sphaericus during vegetative phase of growth. Mtx1 from B. sphaericus 2297 shows higher toxicity against Culex quinquefasciatus larvae than to Aedes aegypti larvae whereas Mtx2 from B. sphaericus 2297 shows lower toxicity against C. quinquefasciatus than to A. aegypti larvae. To test synergism of these toxins against A. aegypti larvae, mtx1 and mtx2 genes were cloned into a single plasmid and expressed in Escherichia coli. Cells producing both Mtx1 and Mtx2 toxins exhibited high synergistic activity against A. aegypti larvae approximately 10 times compared to cells expressing only a single toxin. Co-expression of both toxins offers an alternative to improve efficacy of recombinant bacterial insecticides. There is a high possibility to develop these toxins to be used as an environmentally friendly mosquito control agent.

Cloning and characterization of a mosquito larvicidal toxin produced during vegetative stage of Bacillus sphaericus 2297.

The mosquitocidal toxin 1 (mtx1) gene from genomic DNA of B. sphaericus strain 2297 was cloned and expressed in E. coli. DNA sequencing analysis of the cloned gene revealed a single open reading frame encoding an 870-amino acid polypeptide. Expression level of the full-length gene in E. coli was very low even though strong promoter was used or the gene was expressed as a fusion protein. Expression level was highly improved after the putative leader sequence was deleted, and the truncated gene was expressed as a fusion protein with glutathione S-transferase (GST-tMtx1). E. coli cells expressing GST-tMtx1 was highly toxic to Culex quinquefasciatus larvae and showed lower toxicity against Anopheles dirus and Aedes aegypti larvae. Enterobacter amnigenus An11, a mosquito larval gut colonizable bacteria, transformed with the cloned gene exhibited mosquito larvicidal activity. Result suggested that there is a potential to develop this protein to be used as an alternative mosquito control agent.

Trp132, Trp154, and Trp157 are essential for folding and activity of a Cyt toxin from Bacillus thuringiensis.

Cyt2Aa2 is a cytolytic and mosquito larvicidal toxin produced by Bacillus thuringiensis subsp. darmstadiensis. The toxin contains 3 tryptophan residues at positions 132, 154, and 157. To study the role of tryptophan on protein structure and functions, each tryptophan residue was substituted by phenylalanine and other different amino acids. Expression test in Escherichia coli showed that all mutant proteins were highly produced as inclusion bodies similar to that of the wild type. The mutant W157F showed haemolytic and mosquito larvicidal activities comparable to the wild type but the mutant W157V and all other mutants at positions 132 and 154 have completely lost these activities. Solubilization and proteinase K activation tests indicated that aromatic residue is required at position 157 and tryptophan residues at positions 132 and 154 are critical residues playing important role to maintain structure and functions of the protein and cannot be changed to any other amino acid.

Efficient expression of the mosquito larvicidal binary toxin gene from Bacillus sphaericus in Escherichia coli.

The binary toxin gene encoding BinA (42 kDa) and BinB (51 kDa) from Bacillus sphaericus strain 2297 was cloned and expressed in E. coli. Low expression level was found when both proteins were expressed from a single operon. High expression was observed when the gene encoding an individual protein was placed downstream of the T7 promoter. The expression level of BinB was not different when expressed alone (non-fusion) or as a fusion form with T7 peptide (T7-BinB). Both forms of BinB were equally stable. Unlike BinB, the non-fusion form of BinA was less stable than T7-BinA. The mosquito larvicidal test showed that BinA or BinB alone was not toxic to mosquito larvae, but high toxicity was found when both BinA and BinB were applied. The results suggest that a short peptide of T7 linked to the N-terminus of either BinA or BinB does not affect their toxicity, but may make the toxin, especially BinA, more stable.

A plasmid encoding a combination of mosquito-larvicidal genes from Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus confers toxicity against a broad range of mosquito larvae when expressed in Gram-negative bacteria.

A recombinant plasmid harboring cry4A, cry4B and cry11A from Bacillus thuringiensis subsp. israelensis and binary toxin genes from Bacillus sphaericus has been constructed. The three cry genes were placed under the control of the cry4B promoter whereas the binary toxin gene was controlled by its native promoter. The expression of toxins in Escherichia coli harboring the resulting plasmid, p4BDA-5142, was investigated. Cry4B expression was highest compared to other toxins. Although the level of toxin expression was low compared with E. coli expressing single toxins, the recombinant E. coli strain harboring p4BDA-5142 exhibited broad range mosquito-larvicidal activity against all Aedes, Culex and Anopheles larvae. This work has shown that the development of the recombinant plasmid can be used to broaden the host range spectrum of the appropriate bacterial host for mosquito control.