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Volume 271, Number 24, Issue of June 14, 1996 pp. 14183-14187
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Unusual Amino Acid Determinants of Host Range in the Mtx2 Family of Mosquitocidal Toxins

(Received for publication, November 29, 1995, and in revised form, March 19, 1996)

From the Institute of Molecular and Cell Biology, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Republic of Singapore

Five different mosquitocidal toxin (mtx2) gene homologs have been cloned from eight Bacillus sphaericus strains. Pairwise comparisons of the predicted amino acid sequences show between four and eight substitutions compared with the prototype Mtx2 from B. sphaericus strain SSII-1. Mtx2 from strain SSII-1 was ~7-fold more toxic to Culex mosquito larvae than the Mtx2 homolog from B. sphaericus strain 31-2. Conversely, Mtx2 from strain 31-2 was ~100-fold more toxic to Aedes mosquito larvae than Mtx2 from strain SSII-1. Lys²²⁴ in Mtx2 was found to be the most important amino acid for toxicity to Culex larvae, and substitution of Lys²²⁴ with threonine abolished the toxicity of Mtx2 from strain SSII-1 to these larvae. In complete contrast, Thr²²⁴ was found to be crucial for the toxicity of Mtx2 from strain 31-2 to Aedes larvae, and substitution of Thr²²⁴ with lysine caused a ~100-fold drop in toxicity to these larvae. Thus, amino acid 224 in the Mtx2 family of mosquitocidal toxins is an unusual and important determinant of mosquito larvicidal activity and host range.

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