Bt Research 2025, Vol.16, No.5, 224-233 http://microbescipublisher.com/index.php/bt 225 2 The Molecular Basis of Bt Virulence 2.1 Types and structural characteristics of toxin proteins Bt virulence mainly comes from its toxin protein synthesized during the spore formation stage. The most important ones are the concomitant spore crystal toxins, including two major categories: Cry toxins and Cyt toxins. Cry toxin is the most diverse and most studied delta-endotoxin. It is activated by protease cleavage and acts upon protease cleavage and displays a high specific toxic activity for insects in different orders (Bravo et al., 2017). A typical Cry toxin consists of three domains. Domain I is a helical bundle structure, which can be inserted into the cell membrane of the insect intestinal epithelial cell to form a pore; Domain II and Domain III are β-sheet structures, mediating the specific binding and stability of the toxin to the receptor. Cyt toxin is approximately 27 kDa, which exists alone or forms crystals with Cry, has cell lytic activity and can work together to increase virulence against certain targets. In addition, Bt secretes another type of toxin during the vegetative period, namely the Vip family. Studies have shown that Vip3A can bind to receptors and lead to cell lysis after being activated by protease in the insect midgut, which may form oligomeric pores to play a toxic effect (Figure 1) (Jiang et al., 2020; Gupta et al., 2021). Figure 1 Model representing Vip3A proteins-insecticidal mechanism through pore formation and apoptosis (Adopted fom Gupta et al., 2021) Image caption: (A) The ~62~66 kDa fragments along with ~19~22 kDa fragment can bind with receptors leading to pore formation and cell death. (B) This represents that Vip3A-protoxin can bind to all three specific receptors in the target insect (Adopted fom Gupta et al., 2021) 2.2 Distribution of toxin genes on plasmids A large number of studies have shown that the insecticidal toxin gene of Bt is mainly located on the plasmid it carries. Typical Bt strains (such as the Bacillus thuringiensis HD series) usually contain several plasmids of varying sizes, with large plasmids carrying multiple cry gene clusters. For example, the plasmid pBtoxis (~128kb) of the Israeli variant (Bti) carries a series of toxin genes such as cry4A, cry4B, cry10A, cry11A and cyt, which is the genetic basis for the efficient mosquito-killing virility of this strain. For example, in the Bt kurstaki HD-73 strain, the large plasmid pHT73 contains lepidopteran toxin genes such as cry1Ac, while the other plasmid pHT8 plasmid carries genes such as vip3A, thus conferring the strain on the poisonous effect of a variety of lepidopteran larvae (Chen et al., 2022).
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