Abstract
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.
Highlights
Integrative and conjugative elements (ICEs) are highly prevalent and widely distributed in bacterial genomes [1,2,3]
Some of the most studied ICEs belong to the SXT/R391 family, which are major drivers of multidrug resistance dissemination among various pathogenic Gammaproteobacteria
In silico analyses revealed several putative stabilization systems carried by R391, a prototypical member of the SXT/R391 ICEs family originally isolated from Providencia rettgeri
Summary
Integrative and conjugative elements (ICEs) are highly prevalent and widely distributed in bacterial genomes [1,2,3] Their ability to self-transfer by conjugation between genetically unrelated bacteria contributes to the emergence of multidrug resistant pathogens in diverse taxonomic groups [4,5,6]. Expression of the conjugative function of SXT/ R391 ICEs is tightly regulated by SetR, which represses the expression of the master activator genes setC and setD. Their products activate transcription of int, xis and conjugation-associated operons [14]. Repression of setC and setD is alleviated by induction of the bacterial response to DNA damage, which promotes autoproteolysis of SetR [15]
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