Identification of the origin of transfer (oriT) and a new gene required for mobilization of the SXT/R391 family of integrating conjugative elements

Integrating conjugative elements (ICEs) are self-transmissible, mobile elements that are widespread among bacteria. Following their excision from the chromosome, ICEs transfer by conjugation, a process initiated by a single-stranded DNA break at a specific locus called the origin of transfer (oriT). The SXT/R391 family of ICEs includes SXTMO10, R391, and more than 25 related ICEs found in gammaproteobacteria. A previous study mapped the oriT locus of SXTMO10 to a 550-bp intergenic region between traD and s043. We suspected that this was not the correct oriT locus, because the identical traD-043 region in R391 and other SXT/R391 family ICEs was annotated as a gene of an unknown function. Here, we investigated the location and structure of the oriT locus in the ICEs of the SXT/R391 family and demonstrated that oriT(SXT) corresponds to a 299-bp sequence that contains multiple imperfect direct and inverted repeats and is located in the intergenic region between s003 and rumB'. The oriT(SXT) locus is well conserved among SXT/R391 ICEs, like R391, R997, and pMERPH, and cross-recognition of oriT(SXT) and oriT(R391) by R391 and SXTMO10 was demonstrated. Further-more, we identified a previously unannotated gene, mobI, located immediately downstream from oriT(SXT), which proved to be essential for SXTMO10 transfer and SXTMO10-mediated chromosomal DNA mobilization. Deletion of mobI did not impair the SXTMO10-dependent transfer of the mobilizable plasmid CloDF13, suggesting that mobI has no role in the assembly of the SXTMO10 mating pair apparatus. Instead, mobI appears to be involved in the recognition of oriT(SXT).