Insights into key steps of DNA transfer and the host spectrum of the large conjugative plasmid pXO16 from Bacillus thuringiensis sv. israelensis

Horizontal gene transfer (HGT) plays a crucial role in shaping bacterial populations and communities. Among others, conjugation is a key mechanism of HGT and participates in the transfer of genetic material, mainly plasmids, between mating partners that are in direct contact with one another. As plasmids carry important functions among the species of the Bacillus cereus group, their horizontal displacements among the group members are scrutinised. One of the most studied species of the B. cereus group is Bacillus thuringiensis, well known for its entomocidal toxins and used worldwide as a biopesticide, notably the serovar israelensis, mainly toxic against mosquito larvae. B. thuringiensis sv. israelensis features a large pool of plasmids, including the unique conjugative plasmid pXO16. pXO16 can transfer itself at extremely high frequencies and can efficiently trigger the transfer of virtually any co-resident plasmid or host chromosomal locus. At the molecular level, the mechanism underlying pXO16 conjugation remains rather poorly described. This work therefore aimed at providing new understanding about pXO16 transfer, both regarding its molecular bases and the extent of its transferability and its influence across the B. cereus group. This work first focuses on elucidating key steps and major components of the conjugative apparatus of pXO16. Conjugation implies the action of two main machineries, namely a relaxosome and a transferosome. The transferosome of pXO16 is thought to work as a T4SS (Type IV Secretion System), with some specificities. Here, the importance and activity of key T4SS components are investigated, primarily the conjugative cell wall hydrolase, and the uniqueness of pXO16 relaxosome is reported for the first time. The second part of this work investigates the transfer of pXO16 among the B. cereus group to assess the potential impact of its highly efficient mobilisation properties. While a wide host spectrum is uncovered, the natural distribution
(SC - Sciences) -- UCL, 2022