Bt Research 2025, Vol.16, No.5, 182-193 http://microbescipublisher.com/index.php/bt 184 stage, Bt undergoes changes in cell morphology and membrane composition, involving the remodeling of phospholipids and fatty acid composition to adapt to the formation of spore and spore crystals (Wang et al., 2013). Studies have shown that certain membrane enzymes are upregulated during Bt spore production, which may be used to regulate membrane fluidity and assist in the release of intracellular toxic crystals. These lipopeptides have interface activity and antibacterial activity, which can assist Bt in insect colonization and combat intestinal microbial competition. Although the main insecticidal factor of Bt is proteotoxin, the impact of lipid metabolism on virility is indirect and far-reaching. Membrane lipid composition will affect the configuration and function of toxin receptors on the intestinal cell membrane, and thus affect the efficiency of toxin action (Figure 1) (Šolinc et al., 2023); lipid signaling molecules (such as certain lipopeptides) can act as population sensing signals or regulatory molecules to regulate Bt population behavior and toxin synthesis. In addition, Bt can also synthesize pigment secondary metabolites such as melanin under special environmental conditions. As reported, many Bt strains can produce melanin in the presence of high temperature and L-tyrosine. Figure 1 Visualization of Cyt2Aa oligomers (Adopted from Šolinc et al., 2023) Image caption: (A) 5.6 μM of monomeric Cyt2Aa was subjected to different treatments: Untreated monomer, Cyt2Aa incubated for 1 h with 0.25 mM Brij 35 or with 1% Triton X-100, and incubated for 1 h with 5 and 10 mM DOPC:CHOL (13:1) MLVs (respectively). (B) Cyt2Aa incubated for indicated times with 10 mM DOPC:CHOL (13:1) MLVs. Samples were solubilized with 1% Triton X-100 and incubated at 22 °C. (C) Control: cryo-EM micrograph of DOPC:CHOL (13:1) MLVs in the absence of Cyt2Aa (D) Top: representative micrograph of Cyt2Aa incubated with DOPC:CHOL (13:1) MLVs recorded with cryo-EM (Adopted from Šolinc et al., 2023) 3 The Association Between Bt Toxin Synthesis and Metabolic Pathways 3.1 The synthesis mechanism of toxin protein (Cry, Cyt) The most significant insecticidal factor of Bt is the toxin protein contained in its compost crystals, including Cry toxins represented by the three domain structure and a few Cyt toxins. Cry toxin is a high molecular weight protein specifically expressed during Bt spore production, and the encoding gene is usually located on the plasmid
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