Bt Research 2025, Vol.16, No.5, 224-233 http://microbescipublisher.com/index.php/bt 226 2.3 Molecular mechanisms of the action of toxins and host insects The lethal effect of Bt toxin on target insects is a multi-step molecular process. Taking Cry crystal toxin as an example, after insects eat preparations containing Bt spores and crystals, the crystals dissolve under high pH conditions of the midgut, and the prototoxin protein is activated by the midgut protease to form toxin fragments. The activated Cry toxin monomer then specifically binds to the receptor molecule on the surface of midgut epithelial cells. Studies have shown that different Cry toxins mainly recognize three types of receptors: one is glycoside hydrolase inlaid on the microchorionic, such as aminopeptidase N (APN) and alkaline phosphatase (ALP), which can bind to them with weak reversible binding (Peña-Cardeña et al., 2018); the second is cadherin receptors (Cadherin, such as Bt-R1) on the cell membrane. After the toxin firmly binds to certain segments, it promotes conformational changes in toxin molecules and aggregates into oligomers; the third is ABCC transporter receptors. The latest research has found that binding of certain Cry toxins to transmembrane transporters such as ABCC2 is crucial for pore formation in toxin insertion. When the Cry toxin oligomer is formed, the third step is to insert the membrane of the midgut epithelial cell. The oligomeric toxin molecules form cation-selective pores on the membrane, resulting in permeability of the intestinal epithelial cell membrane and cell swelling and lysis (Gómez et al., 2014). A large number of epithelial cells rupture causes the insect to lose function of the midgut barrier, die from shock or secondary sepsis. 3 Functional Characteristics of Plasmids in Bt 3.1 Types and classification characteristics of Bt plasmids Bt strains usually contain plasmids of varying sizes ranging from small plasmids with thousands of bases to large plasmids with hundreds of kbs. According to the size and function of the plasmid, Bt plasmids can be roughly divided into the following categories, large virulence plasmids: usually >30 kb, carrying one or more insecticidal toxin genes, and are the main plasmids that determine Bt insecticidal activity (Lechuga et al., 2020). Small and medium-sized helper plasmids: 10 kb~30 kb in size, do not contain major toxin genes, but may carry virulence cofactor genes or environmental adaptation-related genes. These plasmids are sometimes called "recessive plasmids" and their removal does not immediately lose insecticidal ability, but may affect the strain's competitive viability. Conjugation plasmid: It carries tra genes related to ligation and can be transferred horizontally between strains. For example, the pXO16 plasmid present in the Bt subspecies Israel can be transferred at high frequency between Bacillus genus, which is an important model for studying the horizontal propagation of Bt plasmids. Embedded plasmids: Some plasmids or a portion thereof can be integrated into chromosomes into proplasmid form (Lechuga et al., 2020). 3.2 Interaction between plasmids and chromosomal genes Although Bt plasmids are independent of chromosome replication, their function interacts closely with chromosomal genes. The expression of toxin genes on plasmids is affected by the global regulatory network encoded by chromosomes. For example, the PlcR transcription factor in Bt is the main control element that regulates a variety of exotoxins. Although it does not directly control Cry toxins, it will regulate various enzymes and toxin release processes. The expression of plasmid toxin genes also depends on the staged σ factor encoded by chromosomes. Cry toxin genes are usually transcriptionized by σ factors such as σ^E and σ^K in the late stage of spore formation. Secondly, plasmids and chromosomes may carry genes with complementary functions and jointly complete a certain physiological function. Plasmids can undergo genetic recombination with chromosomes through integration elements. The common IS sequence of Bt exists both in plasmids and chromosomes, and gene fragments can be exchanged through transposition activities. It has been reported that an embedded phage carried by the plasmid can integrate into specific sites of the Bt chromosome, causing changes in the function of adjacent genes (Méric et al., 2018). In addition, the presence of plasmids also affects the expression of host chromosome genes. Plasmid replication and expression require host resources, so when plasmid load is too high, host cells may
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