A novel T4SS‐mediated DNA transfer used by pXO16, a conjugative plasmid from Bacillus thuringiensis serovar israelensis

Abstract

The entomopathogenic Bacillus thuringiensis serovar israelensis displays peculiar conjugative transfer capabilities, accounted for by the large conjugative plasmid pXO16 (350 kb). The efficient and fast conjugative transfers are accompanied by a macroscopic aggregation of bacterial partners. Moreover, pXO16 has proven capable of effective mobilization and the retro-transfer of both mobilizable and 'non-mobilizable' plasmids. In this work, the aggregation phenomenon is shown to promote pXO16 transfer while not being mandatory for transfer. Transfer of pXO16 to B. thuringiensis recipient strains that do not display aggregation is observed as well, hence enlarging the previously defined host range. The use of variant calling analysis of transconjugants allowed for observation of up to 791 kb chromosomal regions mobilization. Previous analysis of pXO16 did not reveal any Type IV Secretion System (T4SS) homologs, which suggested the presence of an unusual conjugative system. A FtsK/SpOIIIE ATPase gene proved here to be necessary for conjugative transfer. Additionally, the analysis of natural restriction-modification systems in both conjugative partners gave credit to a ssDNA transfer mechanism. A 'transfer israelensis plasmid' (tip) region containing this ATPase gene was shown to code for other potential T4SS proteins, illustrating a conjugative system distantly related to the other known Gram-positive T4SSs.

Insights in a novel gram‐positive type IV secretion system

“…Furthermore, the authors shed light on the architecture, mode of action and genetic structure of this T4SS encoded in the pXO16 plasmid (Makart et al ., ). The previous identification of a putative DNA segregation ATPase FtsK/SpOIIIE (Makart et al ., ) was used as a starting point to define, by bioinformatics tools, a group of 15 genes potentially involved in the T4SS architecture.…”

“…Additionally, the analysis of natural restriction‐modification systems demonstrated a ssDNA transfer mechanism. They were able to assign predicted functions to all of them based on similarity of the genes in databases and a similar configuration to other T4SS (Makart et al ., ). This T4SS would be analogous to those of Tn916, pCW3 and ICEB s1 , systems belonging to the MPF FA T4SS class found in Firmicutes and Actinobacteria (Guglielmini et al ., ).…”

“…As mentioned at the beginning, an unforeseen T4SS encoded in the pXO16 plasmid of Bacillus thuringiensis sv. israelensis has been recently investigated by Makart and colleagues (), and their advances published in this issue of Environmental Microbiology. This bacterium is characterized by its ability to produce and secrete cytosolic or crystal toxins active against insect larvae and nematodes.…”

“…In this issue, Makart and colleagues report a new way of DNA transfer in Gram-positive bacteria based on type IV secretion system (T4SS) (Makart et al, 2018). Despite the need of the complete genetic elucidation of the T4SS encoded in the pXO16 plasmid, Makart and colleagues (2018) shed light in a novel T4SS involved in the horizontal gene transfer between B. thuringiensis sv.…”

“…In this sense, authors hypothesize that pXO16 could initiate transfer from cryptic oriTs in the donor chromosome, as described for R1162 plasmid (Meyer, 2009), and mobilizations would be the result of 'relaxase-in trans mobilization'. Furthermore, the authors shed light on the architecture, mode of action and genetic structure of this T4SS encoded in the pXO16 plasmid (Makart et al, 2018). The previous identification of a putative DNA segregation ATPase FtsK/ SpOIIIE (Makart et al, 2015) was used as a starting point to define, by bioinformatics tools, a group of 15 genes potentially involved in the T4SS architecture.…”