Construction and characterization of a recombinant Bacillus thuringiensis subsp. israelensis strain that produces Cry11B

The mosquitocidal bacterium Bacillus thuringiensis subsp. israelensis (Bti) produces four major endotoxin proteins, Cry4A, Cry4B, Cry11A, and Cyt1A, and has toxicity in the range of many synthetic chemical insecticides. Cry11B, which occurs naturally in B. thuringiensis subsp. jegathesan, is a close relative of Cry11A, but is approximately 10-fold as toxic to Culex quinquefasciatus. To determine whether the addition of Cry11B to Bti would improve its toxicity, we produced this protein in Bti. High levels of Cry11B synthesis were obtained by expression of the cry11B gene under the control of cyt1A promoters and the STAB-SD sequence. This construct was cloned into the shuttle vector pHT3101, yielding the derivative plasmid pPFT11Bs, which was then transformed by electroporation into acrystalliferous (4Q7) and crystalliferous (IPS-82) strains of Bti. Synthesis of Cry11B in Bti 4Q7 produced crystals approximately 50% larger than those produced with its natural promoters without STAB-SD. However, less Cry11B was produced per unit culture medium with this construct than with the wild-type construct, apparently because the latter construct produced more cells per unit medium. Nevertheless, the Bti IPS-82 strain that produced Cry11B with pPFT11Bs was twice as toxic as the parental IPS-82 strain (LC(50) = 1.4 ng/ml versus 3.3 ng/ml, respectively) to fourth instars of C. quinquefasciatus. Against fourth instars of Aedes aegypti, no statistically significant difference between parental Bti IPS-82 (LC(50) = 4.7 ng/ml) and the Bti IPS-82 recombinant producing Cry11B (LC(50) = 3.5 ng/ml) was found in toxicity.