Bt Research 2025, Vol.16, No.5, 182-193 http://microbescipublisher.com/index.php/bt 185 and regulated by the σ factor at the spore production stage. When Bt enters the quiescent phase and begins to form spores, spore-specific transcription factors such as σ^E and σ^K initiate high-level expression of the cry gene, and the folded Cry protein aggregates to form crystals and is encased in the cyst with the spores. This mechanism of synchronous spore production and toxin production ensures that the toxin can enter dormant along with the spores and spread with the spores. During the vegetative period, Cry toxins are generally not expressed or are expressed at very low levels. However, Bt also has the ability to secrete insecticidal proteins during the vegetative phase, such as Vip and Sip proteins. Vip3 proteins are secreted by bacteria into culture medium during the logarithmic growth period of Bt, and are virulent to insects such as Lepidoptera, which is different from the mechanism of action of Cry toxins (Palma et al., 2014; Chen et al., 2022). Sip protein is a protein secreted during the transitional period and is toxic to Coleopteran larvae. 3.2 Energy demand and metabolic support in toxin synthesis The synthesis of insecticidal toxin proteins by Bt is a highly energy-consuming process. It is estimated that Cry toxins may account for more than 20% of the total soluble protein in the bacterial body during the late sporulation stage, and their synthesis requires huge ATP and reducing power (Gong et al., 2012). Therefore, Bt needs to mobilize its metabolic pathways to provide sufficient energy and precursors for toxin synthesis. First, the vigorous operation of sugar metabolism provides ATP for protein biosynthesis. Research shows that by optimizing the medium carbon source and oxygen supply conditions to improve the sugar metabolism rate during Bt fermentation, the final cospore crystal yield of bacteria can significantly increase (López et al., 2023). Secondly, central metabolism requires a large number of amino acids as components of toxin proteins. When the exogenous nitrogen source is insufficient, Bt will initiate its own nitrogen metabolism pathway, including amino acid degradation and mutation, to ensure the supply of raw materials for toxin synthesis. Again, some special metabolic pathways are also related to the efficient expression of toxins. The phenomenon that copper ions promote the synthesis of polyβ-hydroxybutyrate (PHB) in Bt was once thought to increase carbon flow storage, thereby providing a carbon framework and energy for toxin synthesis in the later stage. However, further research found that whether PHB accumulation or not has no direct correlation with toxins and spore formation, and the main effect of Cu2+ still enhances core metabolism and accumulates more raw materials for toxin synthesis. 3.3 Coupling between toxin expression regulation and environmental signal Bt toxin synthesis is strictly regulated at the transcriptional and translational levels, and its temporal and spatial expression is closely related to the environment in which the bacteria are located. The spore production regulation network directly controls the expression of Cry toxin. During the initial stage of sporulation, Spo0A, a key transcription factor in the parent cell, is activated by phosphorylation, which triggers the cascade activation of a series of stage-specific σ factors. Bt also perceives many environmental signals and regulates toxin expression through global regulators. When external carbon sources are abundant, the carbon metabolism repression effect mediated by the carbon metabolism regulator CcpA inhibits the transcription of some sporogenic enzymes and virulence factors. Only when the carbon source is consumed to a certain extent and CcpA inhibition is relieved, can the bacteria enter spore-producing and expressing toxins more actively (López et al., 2023). Some special signaling molecules in the environment can directly influence the toxin synthesis pathway. The study found that the transitional regulatory protein AbrB is very uniquely regulated the expression of a trophic insecticidal protein Sip1Ab1 in Bt. Generally, AbrB is present in large quantities when the nutrients are sufficient and acts as an inhibitor, but in Bt strains, AbrB binds to the sip gene promoter to promote its transcription. This phenomenon shows the sensitivity of Bt regulation network. 4 Secondary Metabolites and Pathogenicity 4.1 Types and characteristics of secondary metabolites (anti-microbial peptides, pigments, etc.) In addition to crystal toxins, Bt can also synthesize a variety of secondary metabolites, including antimicrobial peptides, enzymes, pigments, etc., and has certain biological activity on microorganisms and insects
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