Bt Research 2025, Vol.16, No.6, 269-277 http://microbescipublisher.com/index.php/bt 270 regulates key metabolic and developmental pathways. And to evaluate the evolutionary significance of the dynamic changes of plasmids within the Bacillus cereus group. By integrating insights from genomics and functionology, this study aims to enhance the understanding of Bt adaptability and inform the application strategies of Bt in the fields of biological control and biotechnology. The research results will contribute to a deeper understanding of microbial evolution and the mechanisms driving the metabolic diversity of bacterial pathogens. 2 General Characteristics of Plasmids in Bt Strains 2.1 Common types and size distribution of plasmids in Bt There are indeed many types of plasmids in Bacillus thuringiensis (Bt). Sometimes, five or six strains can be found in one strain, ranging in size from a few kb to over a hundred kb, with a considerable range. Although the distribution of these plasmids is quite diverse, the combination of plasmids among different strains is quite recognizable. Some have even become clues to distinguish subtypes, especially at the level below serotype, and are often used as "typing markers". However, not all serotypes are like this. For instance, the plasmid spectrum of the Israeli type is quite conservative and basically the same. But it's quite different for the Morrison type. There are significant differences among different strains. These plasmids are not only gene "transporters", but also carry many functional segments that are crucial for Bt. For instance, the Cry toxin encoding gene is often attached to these large plasmids. In addition, there are many sequences related to metabolism and physiological functions (Murawska et al., 2014; Bolotin et al., 2017). 2.2 Plasmid copy number, mobility, and stability Not all Bt plasmids have only one or two copies; some can replicate many. This copy number is usually inversely proportional to the size of the plasmid - the smaller the plasmid, the higher the copy number; conversely, larger plasmids have fewer copies. Studies have found that sometimes the total amount of plasmid DNA in Bt bacteria can even exceed chromosomal DNA, indicating that its ability to maintain these exogenous DNA is extraordinary (Zhong et al., 2011). In addition, some plasmids are quite "active", having their own conjugating function and being able to "jump" into other Bt strains or even transfer to other closely related Bacillus species. Thus, horizontal gene transfer occurs. However, these plasmids cannot be kept at will. Their stability depends on the replication control mechanism and the distribution system to be maintained. Their copy numbers actually fluctuate at different growth stages of the bacteria, roughly to strike a balance between maintaining function and conserving resources (Fang and Xu, 2025). 2.3 Interactions between Bt plasmids and chromosomal genomes Bt plasmids and chromosomes do not operate independently but rather have a "back-and-forth" relationship. There are many modular structures on plasmids, which have homology with chromosomal fragments or other plasmid segments. This similarity often stems from historical recombination or transposition events. Some plasmid genes can directly affect gene expression on chromosomes. Conversely, regulatory elements encoded by chromosomes may also determine whether plasmids can stably exist and how much they can replicate (Murawska et al., 2014). This kind of interaction mechanism closely links the functional segments outside the chromosome with the core genome. To some extent, this "coordinated relationship" makes Bt more flexible and resilient in terms of metabolic capacity and environmental adaptability. 3 Plasmid-Encoded Toxins and Pathogenicity Factors 3.1 Plasmid distribution patterns of Cry, Cyt, and Vip toxin genes The toxin genes such as Cry, Cyt and Vip, to put it bluntly, are the "trump cards" of Bt insecticides, but they are not hidden on chromosomes and are mostly concentrated in a few large plasmids. Like the most typical pBtoxis plasmid in the Israeli subspecies Bt strain, not only does it contain multiple Cry and Cyt genes, but these genes are also arranged together like "beads" to form the so-called "pathogenic island", unified transcription, and the killing effect on mosquitoes is very obvious (Zhou et al., 2024). However, Vip toxins, although expressed at different stages and synthesized during the nutritional period, are also present on plasmids. They are not here to "replace"
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