MECHANISM OF ACTION OF BACILLUS-THURINGIENSIS INSECTICIDAL DELTA-ENDOTOXINS

This chapter provides an overview of the biology of Bacillus thuringiensis (Bt). The chapter examines how the recently solved X-ray crystal structure of one Bt toxin allows to model the mode of action of a whole family of related protein toxins, and how a knowledge of the physiology of the insect targets of Bt toxins is needed for a better understanding of the toxic mechanism. Bacillus thuringiensis (Bt), a family of bacteria which make insecticidal proteins, accounts for 90–95% of the insect biocontrol market. This chapter illustrates that Bt is the name given to a family of bacteria found throughout the world. Paradoxically, CryIIIA, the only Bt toxin whose three-dimensional structure has been solved, kills very small insects, and its insolubility at neutral pH makes it difficult to assay in vitro. The crystal structure of CryIIIA permits site-directed mutagenesis and segment swapping experiments to be designed in a rational manner in order to identify the functional domains of the toxins. This chapter explores one attractive prospect that can be the replacement of the specificity domain with a binding domain directed to a target of choice, opening up the possibility of “designer” pesticides or a new class of immunotoxins. Characterization of toxin receptors and investigation of the mechanism of toxin synergism may assist in strategies for resistance management, a vital concern if Bt is to maintain its important position as the most extensively used biological insecticide. © 1994 Academic Press Limited