The spore-forming bacterium, Bacillus thuringiensis(Bt), produces a crystalline parasporal body during sporulation, which in many subspecies contains one or more proteins selectively toxic to insect midgut epithelia. Most of these proteins are protoxins with a molecular mass of about 133,000. When ingested by a susceptible insect, the inclusions dissolve in the midgut juices and are activated by proteolytic cleavage, which releases a toxic peptide of about 65,000. In susceptible insects, this peptide binds to sites on the microvillar membrane, causing cytolysis, apparently by forming transmembrane pores. The cytolysis of midgut cells results in paralysis and subsequent death of the insect. Though less common, naturally truncated protein toxins with masses of about 70,000 also occur. Three major pathotypes of Bt proteins are known: CryI and CryII proteins toxic primarily to lepidopterous insects, CryIII proteins toxic to coleopterous insects, and CryIV proteins toxic to dipterous insects. The genes encoding more than 50 Bt crystal proteins have been cloned and sequenced and several of these have already been used to construct recombinant microbial insecticides and transgenic organisms including viruses, algae, and insect-resistant plants. Analysis of Bt genes indicate that the protein toxins they encode consist of at least two functional domains: a series of five blocks of conserved amino acids that comprise the structural core of the molecule including the putative toxic moiety, and a hypervariable region thought to define the spectrum of activity. Determination of the mode of action at the molecular level and the genetic basis for insect specificity should enable recombinant DNA technology to be used to expand the insect host range of Bt as well as increase its toxicity against insects. Moreover, the high toxicity and specificity of the insecticidal proteins produced by Bt and their action on midgut microvilli suggest that other types of peptides or simple organic molecules could be designed as selective insecticides to attack other midgut functions such as ion channels, peptide and amino acid transport systems, and midgut proteases.
