Bt Research 2024, Vol.15, No.3, 154-163 http://microbescipublisher.com/index.php/bt 156 2.2 Chromosomal organization The chromosomal organization of Bt is characterized by a single circular chromosome that encodes essential genes for the bacterium's survival and pathogenicity. For example, the complete genome sequence of Bt HER1410 reveals a circular chromosome that contains a unique cry gene, cry1Ba4, located in a genomic island near the chromosome replication origin (Figure 2) (Lechuga et al., 2020a). This chromosomal arrangement is crucial for the regulation of gene expression and the maintenance of genomic stability. Additionally, the chromosome often contains genes involved in metabolic pathways, antibiotic resistance, and other essential cellular functions (Liu et al., 2017; Lechuga et al., 2020a). Figure 2 Circular representation of B. thuringiensis HER1410 chromosome (Adopted from Lechuga et al., 2020a) 2.3 Plasmid content Plasmids play a significant role in the genomic architecture of Bt, often carrying genes that encode insecticidal toxins and other virulence factors. The number and size of plasmids can vary widely among different Bt strains. For instance, Bt strain HD521 contains six circular plasmids, while strain HS18-1 has nine circular plasmids (Sun et al., 2021). These plasmids encode various virulence proteins, including Cry and Vip toxins, which are critical for the bacterium's insecticidal activity (Sun et al., 2021). In Bt GR007, three megaplasmids were identified, with the largest two (pGR340 and pGR157) containing multiple pesticidal protein genes, such as cry and vip genes (Pacheco et al., 2021). Similarly, Bt HER1410 harbors two megaplasmids, pLUSID1 and pLUSID2, which are involved in virulence and sporulation processes, respectively (Lechuga et al., 2020a). The presence of plasmids also facilitates horizontal gene transfer, contributing to the genetic diversity and adaptability of Bt. For example, the comparative genomics of Bt subsp. israelensis revealed active plasmid exchange among strains, highlighting the dynamic nature of plasmid content in this species (Bolotin et al., 2017). This exchange of genetic material allows Bt to acquire new traits, such as resistance to environmental stresses and enhanced pathogenicity, thereby increasing its ecological fitness and effectiveness as a biopesticide (Bolotin et al., 2017; Méric et al., 2018).
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