The effect of chemical modification of aromatic and basic residues of the Bacillus thuringiensis insecticidal CrylA(c) toxin on toxicity and receptor binding was studied. Modification of four or more of the 43 arginine residues resulted in abolition of toxicity towards Manduca sexfa and Pieris brassicae in vivo and a Choristoneura fumiferana cell line in witro. Modification of seven or more of the 27 tyrosine residues resulted in reduction of toxicity. Upon modification of most of the 10

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... han residues, toxicity was reduced. Modification of histidine residues had no effect on toxicity. A quantitative binding assay was developed and optimized to compare the binding of derivatives with native toxin t o M. sexta brush border membrane vesicles. The reduction or abolition of toxicity observed upon selective modification of tyrosine or arginine residues was reflected in the binding abilities of the derivatives. However a non-toxic derivative, modified at tryptophan residues, retained its ability to bind vesicles. These results suggest that tyrosine and arginine residues are involved in the binding interaction of toxin with receptor but tryptophan residues are involved in some subsequent step.