Identification of the Origin of Transfer (<i>oriT</i>) and a New Gene Required for Mobilization of the SXT/R391 Family of Integrating Conjugative Elements

<jats:title>ABSTRACT</jats:title><jats:p>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 (<jats:italic>oriT</jats:italic>). The SXT/R391 family of ICEs includes SXT<jats:sup>MO10</jats:sup>, R391, and more than 25 related ICEs found in gammaproteobacteria. A previous study mapped the<jats:italic>oriT</jats:italic>locus of SXT<jats:sup>MO10</jats:sup>to a 550-bp intergenic region between<jats:italic>traD</jats:italic>and s043. We suspected that this was not the correct<jats:italic>oriT</jats:italic>locus, because the identical<jats:italic>traD</jats:italic>-s043 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<jats:italic>oriT</jats:italic>locus in the ICEs of the SXT/R391 family and demonstrated that<jats:italic>oriT</jats:italic><jats:sub>SXT</jats:sub>corresponds to a 299-bp sequence that contains multiple imperfect direct and inverted repeats and is located in the intergenic region between s003 and<jats:italic>rumB</jats:italic>′. The<jats:italic>oriT</jats:italic><jats:sub>SXT</jats:sub>locus is well conserved among SXT/R391 ICEs, like R391, R997, and pMERPH, and cross-recognition of<jats:italic>oriT</jats:italic><jats:sub>SXT</jats:sub>and<jats:italic>oriT</jats:italic><jats:sub>R391</jats:sub>by R391 and SXT<jats:sup>MO10</jats:sup>was demonstrated. Furthermore, we identified a previously unannotated gene,<jats:italic>mobI</jats:italic>, located immediately downstream from<jats:italic>oriT</jats:italic><jats:sub>SXT</jats:sub>, which proved to be essential for SXT<jats:sup>MO10</jats:sup>transfer and SXT<jats:sup>MO10</jats:sup>-mediated chromosomal DNA mobilization. Deletion of<jats:italic>mobI</jats:italic>did not impair the SXT<jats:sup>MO10</jats:sup>-dependent transfer of the mobilizable plasmid CloDF13, suggesting that<jats:italic>mobI</jats:italic>has no role in the assembly of the SXT<jats:sup>MO10</jats:sup>mating pair apparatus. Instead,<jats:italic>mobI</jats:italic>appears to be involved in the recognition of<jats:italic>oriT</jats:italic><jats:sub>SXT</jats:sub>.</jats:p>