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Bacillus thuringiensis insecticidal toxins: Biochemical and physiological properties
Abstract
The Gram positive, spore-forming bacterium, Bacillus thuringiensis produces insecticidal protein toxins, which are remarkably specifically toxic to insects, but not to humans and other vertebrates. The toxins, contained within the parasporal crystals, are solubilized in insect midguts after crystal ingestion. The toxins are activated from protoxins by insect gut proteases. The activated toxins bind to their receptor proteins present on the brush-border membrane of the insect midgut epithelium and form pores within the brush-border membrane. This disrupts insect gut function, stops insect feeding and eventually kills the insect. To facilitate understanding of the interaction between the toxin and insect host, I have examinated the biochemical and physiological characteristics of the toxins: (1) The toxin in crystal form is sensitive to UVA and UVB fractions in the sunlight. Sunlight irradiation causes inactivation of the toxins. The mechanism is unknown. My data suggest that the mechanism involves a cystine (disulfide) direct photolysis process. The intermolecular disulfides among toxin subunits are twisted from the most stable conformation as evidenced from solubility analyses of toxin crystals; (2) The toxin molecule consists of three domains in tertiary structure. The function of one of these domains, Domain 3, has been less clear than others. During my research I found that HD-73 toxin binds to biotin-containing proteins. The binding is not inhibited by free biotin or biocytin, but is inhibited by other biotin-containing proteins. Sequence comparisons suggest that Domain 3 may be the biotin recognition region; (3) The results from (2) imply the biotin-containing proteins in insect hosts may play an important role in Bt toxicity. The biotin-protein profile in brush-border membrane in the tobacco hornworm, Manduca sexta, shows two or three major proteins present. Their possible roles are discussed. Avidin, a natural biotin-binding protein, was also investigated for insecticidal activity and found to be a potential insecticidal agent. (4) The physiological roles of the toxin present in Bt spores during sporulation and germination are assessed by comparing the crystal$\sp+$ and crystal$\sp-$ strains. A spore coat model is proposed to explain the unique arrangement of toxin in spore coat.
Subject Area
Microbiology|Biochemistry|Entomology
Recommended Citation
Du, Cheng, "Bacillus thuringiensis insecticidal toxins: Biochemical and physiological properties" (1996). ETD collection for University of Nebraska-Lincoln. AAI9623621.
https://digitalcommons.unl.edu/dissertations/AAI9623621