Common features of Bacillus thuringiensis toxins specific for Diptera and Lepidoptera

Abstract

The complete nucleotide sequence of a cloned gene encoding a 130‐kDa crystal protein of Bacillus thuringiensis (B.t.) subspecies israelensis has been determined. The recombinant protein (Bt8) was purified and shown to be a mosquito‐specific toxin with a LC 50 value of 43 ng/ml to third‐instar larvae of Aedes aegypti. Bt8 is processed by proteases or midgut extracts of mosquito larvae into toxic fragments of 68–78 kDa. Deletion mapping indicated that the active fragment of Bt8 is localized in the N‐terminal half of the protoxin molecule. The deduced amino acid sequence of Bt8 has been compared with that of Bt2, a Lepidoptera‐specific toxin, previously cloned from Bacillus thuringiensis berliner. Highly homologous amino acid stretches are present in the C‐terminal half of the proteins. The N‐terminal parts show much less sequence homology but they display a strikingly similar distribution of hydrophilic and hydrophobic amino acids. In addition, Bt8 and Bt2 show a significant immunological cross‐reaction. The data indicate that although these B.t. delta endotoxins exhibit a different insect‐host specificity, they are structurally related and might use a similar mechanism to interact with insect cell membranes. Copyright © 1988, Wiley Blackwell. All rights reserved