Bt Research 2024, Vol.15, No.3, 154-163 http://microbescipublisher.com/index.php/bt 158 plasmid environment, which includes mobile genetic elements that contribute to the genetic diversity and adaptability of the bacterium (Fayad et al., 2020). This organization allows for the efficient production of multiple toxins, enhancing the overall insecticidal activity. 4.3 Regulatory mechanisms of toxin genes The expression of Cry and Cyt toxin genes is tightly regulated to ensure their effective production during sporulation. Various regulatory proteins and environmental factors influence the transcription of these genes. For instance, the presence of a 20-kDa protein has been shown to enhance the production of CytA protein in Escherichia coli, although it is not required in Bacillus thuringiensis. Additionally, the interaction between Cry and Cyt toxins can be modulated by specific mutations in their receptor-binding domains, which affect their binding affinity and synergistic interactions (Lailak et al., 2013). Understanding these regulatory mechanisms is crucial for optimizing the production of Bt toxins and improving their efficacy as bioinsecticides. 5 Virulence Factors and Pathogenicity Islands 5.1 Identification of virulence genes Virulence genes in Bacillus thuringiensis (Bt) are crucial for its pathogenicity and are often located on mobile genetic elements such as plasmids and transposons. For instance, the study by (Wang et al., 2008) identified a locus in a virulence-attenuated Bt mutant that encodes 29 potential protein-coding ORFs, many of which share homology with genes on the plasmid pE33L466 of Bacillus cereus. This locus includes mobile elements like the transposon Tn4430, which plays a significant role in Bt virulence during Manduca sexta infection. Additionally, the yqgB and yqfZ genes have been identified as important for Bt's pathogenicity, as their simultaneous inactivation results in attenuated virulence against Bombyx mori larvae (Wang et al., 2008). 5.2 Pathogenicity islands Pathogenicity islands (PAIs) are distinct genetic elements that encode various virulence factors and are typically absent in non-pathogenic strains. PAIs are a subclass of genomic islands acquired through horizontal gene transfer and contribute significantly to the virulence of bacterial pathogens (Gal-Mor and Finlay, 2006). In Bt, PAIs have been identified that contain genes encoding pesticidal proteins and other virulence factors. For example, the complete genome sequencing of Bt GR007 revealed multiple pesticidal protein genes located on megaplasmids, which are likely part of PAIs (Pacheco et al., 2021). These PAIs contribute to the rapid evolution and diversification of Bt, enabling it to adapt to different hosts and environments (Desvaux et al., 2020). 5.3 Horizontal gene transfer Horizontal gene transfer (HGT) is a key mechanism in the evolution of microbial genomes, including the acquisition of virulence factors in Bt. HGT allows for the transfer of PAIs and other mobile genetic elements between different bacterial species, facilitating the spread of virulence genes (Dobrindt et al., 2004). The study of the conjugative plasmid pAW63 in the Bacillus cereus group, which includes Bt, has provided insights into the genesis of virulence plasmids such as pXO2 in Bacillus anthracis and pBT9727 in Bt. These plasmids share a common backbone and exhibit regions of high sequence plasticity, indicating their evolution through HGT (Auwera et al., 2005). Additionally, the presence of mobile elements like transposons and insertion sequences in Bt further supports the role of HGT in its virulence (Wang et al., 2008). In summary, the identification of virulence genes, the role of pathogenicity islands, and the impact of horizontal gene transfer are critical components in understanding the genomic architecture and pathogenicity of Bacillus thuringiensis. These elements collectively contribute to the bacterium's ability to cause disease and adapt to various environments. 6 Mobile Genetic Elements 6.1 Plasmids Plasmids play a crucial role in the genetic architecture of Bacillus thuringiensis (Bt), contributing to its adaptability and pathogenicity. The modular genetic architecture of plasmids such as pIS56-63, which harbors the cry1Ab21 gene, highlights their role in toxin production and gene transfer. This plasmid is composed of four functional modules: a mobile insertion cassette, a putative conjugative element, a regulation sequence, and a
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