Localized mutagenesis defines regions of the Bacillus thuringiensis delta-endotoxin involved in toxicity and specificity.
Bacillus thuringiensis produces a variety of delta-endotoxins which bind to specific receptors in insect larval midguts. Following insertion into the membrane there is an alteration of ion flux culminating in osmotic lysis. Mutagenic oligonucleotides were used to define regions in one of these toxins involved in specificity and toxicity. One region is highly conserved among all toxins sequenced to date and many mutations resulted in loss of toxicity for three test Lepidoptera. The mutant toxins had lost the capacity to inhibit K(+)-dependent amino acid transport into larval midgut vesicles, but there was no effect on their ability to compete with wild type toxin for binding. The results are consistent with this amphiphilic helical region of the toxin being essential for toxicity. A second mutagenized region overlapped a portion of another potential amphiphilic helix. Mutations of only 2 residues, Ala-92 and Arg-93, resulted in loss of toxicity for two lepidopteran larvae but some activity remained for a third. The A92D mutant toxin competed with the wild type toxin for binding to vesicles prepared from midguts from the sensitive but not from the insensitive larvae. Decreased toxicity was also found when this mutation was transferred to two other related protoxin genes. A number of mutations of each of these residues was analyzed and selective loss of toxicity correlated with the absence of a positive charge. Despite being distal from the presumptive specificity domain, 1 or both of these residues must have an important role in the specific binding of toxins.