Bt Research 2025, Vol.16, No.6, 269-277 http://microbescipublisher.com/index.php/bt 272 Figure 1 The rap63-phr63 transcription unit (Adopted from Cardoso et al., 2020) Image caption: (A) Schematic representation of the rap63-phr63 locus in the pAW63 plasmid. Arrowheads correspond to the primers used to amplify the three RT-PCR fragments (fragments A, B, and C). (B) RT-PCR experiment. Total RNA was extracted from a t3 culture in HCT medium at 37 °C (Adopted from Cardoso et al., 2020) 4.3 Plasmid functions related to energy metabolism For metabolism to run smoothly, the energy system must not fail. Bt also has plasmid output in this aspect. They encode some functional modules related to fermentation and amino acid metabolism, enabling Bt to "switch routes" and continue to operate when facing different resource conditions. Especially during state transitions such as the formation of spores or the establishment of biofilms, energy distribution must be precise. At this point, the plasmid regulatory system, such as Rap-Phr, not only regulates structural occurrence but also affects energy usage patterns to a certain extent (Fazion et al., 2018). These regulatory methods, although not necessarily directly determining whether Bt can successfully infect the host, at least determine whether its "stamina" is sufficient when it enters the host or environment and survives for a long time. 5 Relationship Between Plasmids and Bt’s Environmental Adaptability 5.1 Plasmid contributions to tolerance of extreme environments (e.g., high salt, heat) The survival of Bt in "difficult" environments such as high salt and high temperature is most likely related to plasmids. Many Bt strains, especially the Israeli type, have an astonishingly large number of plasmids. This rich plasmid library, at least in terms of genetic flexibility, is an underlying advantage for it to adapt to various environmental stresses (Gillis et al., 2018). More interestingly, some plasmids also carry quorum sensing regulatory modules like Rap-Phr, which can regulate key physiological processes such as spore formation and biofilm construction (Cardoso et al., 2020). These mechanisms may not directly resist heat and salt, but they might enable Bt to survive and get through the drastic environmental changes through "detour regulation".
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