Bt Research 2024, Vol.15, No.2, 76-86 http://microbescipublisher.com/index.php/bt 79 Figure 2 Schematic representation of the mechanism of action of 3d-Cry toxins in Lepidoptera at the cellular level, showing the immunolocalization of Cry toxin during intoxication (Adopted from Pardo-López et al., 2013) Image caption: It illustrates the cellular mechanism of action of 3d-Cry toxins in Lepidoptera. The process begins with the ingestion of Cry proteins by larvae, leading to the solubilization and activation of protoxins by midgut proteases. The activated toxin binds to receptors located in the apical microvilli of insect midgut cells. Subsequently, the toxin inserts itself into the apical membrane, forming pores that result in cell death. The schematic includes immunolocalization images of Cry toxin within the insect midgut cells, highlighting the progression from ingestion to cellular intoxication (Adopted from Pardo-López et al., 2013) 4 Genetic and Molecular Mechanisms 4.1 Gene structure and organization The gene structure and organization of plasmid-encoded toxins in Bacillus thuringiensis (Bt) are complex and diverse. For instance, the plasmid pTAND672-2 fromB. thuringiensis serovar israelensis carries mosquitocidal toxin genes along with genes for conjugation and recombination, forming a novel integrative and conjugative element (ICE) (Geng et al., 2023). This plasmid integrates into the chromosome of Lysinibacillus sphaericus through site-specific recombination mediated by a tyrosine integrase, Int143 (Geng et al., 2023). Another example is the plasmid pH3-180 in the novel Bt strain H3, which contains 11 novel Cry toxin genes organized in an orf1-gap-orf2 structure, highlighting the dynamic nature of toxin gene organization in Bt plasmids (Fayad et al., 2020). Additionally, the large plasmid pBtoxis in B. thuringiensis subsp. israelensis encodes all insecticidal toxins and includes 125 potential coding sequences, many of which are involved in gene regulation and physiological processes (Stein et al., 2006; Zhang et al., 2017). 4.2 Regulation of toxin gene expression The regulation of toxin gene expression in Bacillus thuringiensis involves multiple mechanisms. The transcription of cry genes, which encode crystal proteins responsible for insecticidal activity, can be dependent or independent of sporulation (Lereclus et al., 2000). The pleiotropic regulator PlcR activates the transcription of various genes encoding extracellular proteins, including toxins, and its expression is controlled by the transition state regulator SpoOA (Lereclus et al., 2000). In the case of the plasmid pBtoxis, transcriptional analysis revealed that 29 out of 40 surveyed coding sequences were transcribed, including those with similarities to known transcriptional regulators, suggesting a complex regulatory network influencing toxin gene expression (Stein et al., 2006).
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