Many strains within the 22 serotypes of Bacillus thuringiensis produce crystal delta-endotoxins with slight differences in their insecticidal toxicity spectrum in vivo. Since the basis of this specificity is unknown, we chose to compare the activity of delta-endotoxins from three strains: B. thuringiensis var. kurstaki HD-1, var. aizawai HD-249 and var. thuringiensis HD-350, both in vivo and on insect cell lines in vitro. Immunoblotting with antisera to activated var. kurstaki P1 lepidopteran toxin revealed antigenic cross-reaction with the 130 X 10(3) Mr toxin of var. aizawai, and with polypeptides of 130 and 138 (X 10(3)) Mr from var. thuringiensis. In addition, crystals from var. kurstaki and var. aizawai contained an antigenically related 63 X 10(3) Mr protein that did not cross-react with antisera to the 130 X 10(3) Mr component. Bioassays on Pieris brassicae larvae (Lepidoptera) and Aedes aegypti larvae (Diptera) indicated that the 130 X 10(3) Mr protein of var. kurstaki, and the 138 plus 130 X 10(3) Mr components of var. thuringiensis killed only P. brassicae, while the 130 X 10(3) Mr protein of var. aizawai and the 63 X 10(3) Mr proteins of var. aizawai and var. kurstaki were toxic to both P. brassicae and A. aegypti. Activation of the 130 and 138 (X 10(3)) Mr proteins of the three varieties of B. thuringiensis with insect gut proteases yielded active products of 50–60 (X 10(3)) Mr. Assay of these products on a range of lepidopteran and dipteran cell lines revealed very different toxicity spectra: var. kurstaki killed only one lepidopteran line, var. thuringiensis killed two lepidopteran lines, while var. aizawai was cytolytic to all of the lepidopteran and most of the dipteran cell lines tested, reflecting its broader spectrum in vivo. Thus we have shown that antigenic cross-reaction of B. thuringiensis delta-endotoxins does not necessarily imply a similar toxicity spectrum in vivo or in vitro.

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