pXO16 from Bacillus thuringiensis serovar israelensis: almost 350 kb of Terra incognita?
(2014) International Society for Plasmid Biology Scientific Meeting 2014 — Location: Palm Cove, Australia (27.October.2014)
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- The conjugative plasmid pXO16 (350 kb) from Bacillus thuringiensis serovar israelensis encodes an aggregation-mediated conjugation system and is able of transferring itself and other non-conjugative and non-mobilisable plasmids in a fast and very efficient manner. Even though its conjugative kinetics and capabilities have been extensively studied, the genetic bases for this unique transfer system remain largely unknown. In this study, CDS were predicted from the plasmid sequence and their putative functions were examined with a particular attention to possible conjugative and aggregation functions. Sequence analysis of pXO16 displayed unexpected characteristics for a conjugative plasmid: its 467 putative CDS (88% of its sequence) are all in the same orientation and more than two third of them showed no homologue in the databases. These features are usually shared by jumbo phages. Concerning conjugative functions, no significant type IV secretion system homologues were found, strongly suggesting that pXO16 encodes an unforeseen conjugative system. Using double-recombination mutagenesis, knockout of three selected regions was performed: a 25-kb region including 5 cell-surface-associated CDS, an FtsK-like coding ORF and the gene coding for a bona fide S-layer protein. Transfer frequencies and aggregation phenotype of these mutants were then analyzed, together with the modification of their host cell surface. The 25-kb cell-surface-associated knockout mutant was not able to produce macroscopic aggregation but transfer was still detected, yet at lower frequencies (1,000-fold for conjugative transfer and 100-fold for mobilization). This proves that aggregation is important but not essential in pXO16 transfer. The other mutant phenotypes and their surface properties will also be discussed in more details and should give more insights on the conjugative mechanism encoded by pXO16.
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Makart, L., Gillis, A., & Mahillon, J. (2014). pXO16 from Bacillus thuringiensis serovar israelensis: almost 350 kb of Terra incognita? International Society for Plasmid Biology Scientific Meeting 2014, Palm Cove, Australia. https://hdl.handle.net/2078.5/195012