Effect of the carboxyl-terminal portion of Cry1Ab in Bacillus thuringiensis on toxicity against the silkworm, Bombyx mori

Bacillus thuringiensis strain AF101 produces the Cry1Ab type of insecticidal crystal proteins (ICPs) which are slightly toxic to the silkworm, Bombyx mori. Two chimeric cry1Ab genes were constructed using the cry1Ab genes of B. thuringiensis strain AF101 and serovar kurstaki HD-1 by exchanging their coding regions at the carboxyl-terminal portion of the insecticidal crystal protein (ICP). The effect of the carboxyl-terminal portion in the ICP on toxicity against the silkworm larvae, Bombyx mori, was examined using chimeric Cry1Ab ICPs produced in Escherichia coli transformants harboring the constructed ICP genes. Though no remarkable change in insecticidal activity was observed between parental and chimeric AF101 ICPs, the toxicity of the chimeric HD-1 ICP against the silkworm larvae was about one-fourth that of the parental ICP. These results indicate that the carboxyl-terminal portion in the AF101 Cry1Ab ICP, known as the non-toxin region, also affected the toxicity of B. thuringiensis against the silkworm larvae.