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Integrative and Conjugative Elements (ICEs) of the SXT/R391 family disseminate multidrug resistance among pathogenic  Gammaproteobacteria  such as  Vibrio cholerae . SXT/R391 ICEs are mobile genetic elements that reside in the chromosome of their host and eventually self-transfer to other bacteria by conjugation. Conjugative transfer of SXT/R391 ICEs involves a transient extrachromosomal circular plasmid-like form that is thought to be the substrate for single-stranded DNA translocation to the recipient cell through the mating pore. This plasmid-like form is thought to be non-replicative and is consequently expected to be highly unstable. We report here that the ICE R391 of  Providencia rettgeri  is impervious to loss upon cell division. We have investigated the genetic determinants contributing to R391 stability. First, we found that a  hipAB -like toxin/antitoxin system improves R391 stability as its deletion resulted in a tenfold increase of R391 loss. Because  hipAB  is not a conserved feature of SXT/R391 ICEs, we sought for alternative and conserved stabilization mechanisms. We found that conjugation itself does not stabilize R391 as deletion of  traG , which abolishes conjugative transfer, did not influence the frequency of loss. However, deletion of either the relaxase-encoding gene  traI  or the origin of transfer ( oriT ) led to a dramatic increase of R391 loss correlated with a copy number decrease of its plasmid-like form. This observation suggests that replication initiated at  oriT  by TraI is essential not only for conjugative transfer but also for stabilization of SXT/R391 ICEs. Finally, we uncovered  srpMRC , a conserved locus coding for two proteins distantly related to the type II (actin-type ATPase)  parMRC  partitioning system of plasmid R1. R391 and plasmid stabilization assays demonstrate that  srpMRC  is active and contributes to reducing R391 loss. While partitioning systems usually stabilizes low-copy plasmids,  srpMRC  is the first to be reported that stabilizes a family of ICEs.

plos genetics

Replication and Active Partition of Integrative and Conjugative Elements (ICEs) of the SXT/R391 Family: The Line between ICEs and Conjugative Plasmids Is Getting Thinner