Bt Research 2024, Vol.15, No.3, 154-163 http://microbescipublisher.com/index.php/bt 157 In summary, the genomic structure of Bacillus thuringiensis is characterized by a circular chromosome and multiple plasmids, which together encode a wide array of genes responsible for the bacterium's insecticidal properties and adaptability. The variability in genome size, chromosomal organization, and plasmid content among different Bt strains underscores the genetic diversity and evolutionary potential of this important biopesticide. 3 Functional Elements in the Bt Genome 3.1 Protein-coding genes Bacillus thuringiensis (Bt) is renowned for its diverse array of protein-coding genes, particularly those encoding insecticidal proteins. The genome of Bt isolate T414, for instance, contains numerous protein-coding sequences, including genes for parasporal crystal proteins such as cry1Aa, cry1Ab, cry1Ac, cry1IAa, cry2Aa, cry2Ab, and cyt1, as well as the vegetative insecticidal protein gene vip3Aa. These genes are primarily plasmid-borne, located on plasmids p414A and p414E (Reyaz et al., 2019). Similarly, the Bt GR007 strain harbors multiple pesticidal protein genes, including 10 cry genes (e.g., cry1Ab, cry1Bb, cry1Da) and two vip genes (vip3Af and vip3Ag), distributed across its chromosome and three megaplasmids (Pacheco et al., 2021). These protein-coding genes are crucial for Bt's biopesticidal properties, enabling it to target a wide range of insect pests. 3.2 Regulatory elements Regulatory elements in the Bt genome play a pivotal role in the expression and regulation of its protein-coding genes. These elements include promoters, enhancers, and other sequences that control the transcription and translation of genes. The presence of these regulatory elements ensures that the insecticidal proteins are produced in response to specific environmental cues, optimizing the bacterium's effectiveness as a biopesticide. For instance, the intrinsic promoters associated with the Cry proteins in Bt GR007 facilitate the expression of these proteins in an acrystalliferous strain, demonstrating the importance of regulatory elements in gene expression (Pacheco et al., 2021). 3.3 Non-coding RNAs Non-coding RNAs (ncRNAs) are another significant component of the Bt genome, contributing to the regulation of gene expression and adaptation to environmental changes. A comparative study of ncRNAs in Bacillus cereus sensu lato, which includes Bt, revealed a diverse set of ncRNAs across different strains. The most prevalent functional category of ncRNAs was Cis-reg, with Riboswitch being the most frequent class in chromosomes, while Group II introns were more common in plasmids (Gonçalves et al., 2021). The wide distribution and diversity of ncRNAs in Bt suggest their role in enhancing the bacterium's adaptability and regulatory capabilities, which are essential for its survival and efficacy as a biopesticide. 4 Toxin Genes and Their Organization 4.1 Cry and Cyt toxin genes Bacillus thuringiensis (Bt) produces a variety of Cry and Cyt toxins, which are crucial for its insecticidal properties. Cry toxins, such as Cry4Aa, Cry4Ba, Cry10Aa, and Cry11Aa, and Cyt toxins, such as Cyt1Aa and Cyt2Ba, are particularly effective against mosquito larvae (Pérez et al., 2005; Fernández-Luna et al., 2010). The Cry toxins function by binding to specific receptors in the insect midgut, leading to pore formation and cell lysis (Pardo-López et al., 2013). Cyt toxins, on the other hand, can act as receptors for Cry toxins, enhancing their binding and toxicity (Pérez et al., 2005). For instance, Cyt1Aa has been shown to synergize with Cry11Aa by functioning as a membrane-bound receptor, thereby overcoming insect resistance (Pérez et al., 2005). 4.2 Gene clusters and operons The genes encoding Cry and Cyt toxins are often organized in clusters and operons, facilitating their coordinated expression. For example, the cryIVD and cytA genes are located within a 9.4-kb HindIII fragment and are co-expressed in Bacillus thuringiensis, leading to synergistic toxicity against mosquito larvae. Additionally, novel strains of Bt, such as the H3 strain, have been found to carry multiple Cry toxin genes organized in a dynamic
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