The insecticidal CrylB crystal protein of Bacillus thuringiensis ssp. thuringiensis has dual specificity to coleopteran and lepidopteran larvae

The crystals found in sporulation extracts of Bacillus thuringiensis (Berliner) contain proteins that are highly toxic to insects. Different crystal proteins exhibit distinct specificities for restricted groups of insects. An uncharacterized strain of B. thuringiensis (BtS2), derived from China, was found to carry several crystal protein genes and to be toxic to a wide variety of insects, including some coleopterans. Surprisingly, the coleopteran toxicity was traced to a CryIB-class protein. The previously cloned CryIB protein from B. thuringiensis ssp. thuringiensis strain HD-290-1, which was believed to be lepidopteran-specific, was also found to be toxic to at least two species of coleopteran larvae under certain conditions. In contrast to CryIB toxicity toward lepidopterans, the coleopteran activity of CryIB is enhanced by solubilization and by truncation with trypsin prior to administration. The magnitude of this effect varies with the host species and is reversed for the one lepidopteran tested. These results suggest that, for at least some insects, the apparent host specificity of CryIB may depend both on differences in midgut environment and on differences in toxin-receptor interaction. The results of insect toxicity experiments with a series of deletion mutants allowed definition of a CryIB protein fragment of ca. 65 kDa as the smallest peptide that retains bioactivity against both lepidopteran and coleopteran larvae. Deletions smaller than this resulted in the production of a protein that was nontoxic to both lepidopteran and coleopteran larvae