Comparative analysis of MOBQ4plasmids demonstrates that MOBQis acis-acting-enriched relaxase protein family

ABSTRACT A group of small mobilizable plasmids is increasingly being reported in epidemiology surveys of enterobacteria. Some of them encode colicins, while others are cryptic. All of them encode a relaxase belonging to a previously non-described group of the MOB Q class, MOB Q4 . While highly similar in their mobilization module, two families with unrelated replicons can be distinguished, MOB Q41 and MOB Q42 . Members of both groups were compatible between them and stably maintained in E. coli . MOB Q4 plasmids were mobilized by conjugation. They contain two transfer genes, mobA coding for the MOB Q4 relaxase and mobC , which was non-essential but enhanced the plasmid mobilization frequency. The origin of transfer was located between these two divergently transcribed mob genes. MPF I conjugative plasmids were the most efficient helpers for MOB Q4 conjugative transmission. No interference in mobilization was observed when both MOB Q41 and MOB Q42 were present in the same donor cell. Remarkably, MOB Q4 relaxases exhibited a cis -acting preference for their oriT s, a feature already observed in other MOB Q plasmids. These findings indicate that MOB Q4 plasmids can efficiently spread among enterobacteria aided by coresident IncI1, IncK and IncL/M plasmids, while ensuring their self-dissemination over highly-related elements. IMPORTANCE Plasmids are key vehicles of horizontal gene transfer and contribute greatly to bacterial genome plasticity. A group of plasmids, called mobilizable, is able to disseminate aided by helper conjugative plasmids. Here, we studied a group of phylogenetically-related mobilizable plasmids, MOB Q4 , commonly found in clinically-relevant enterobacteria, uncovering the helper plasmids responsible for their dissemination. We found that the two plasmid species encompassed in the MOB Q4 group can coexist and transfer orthogonally, despite origin-of-transfer cross-recognition by their relaxases. Specific discrimination among their highly similar oriT sequences is guaranteed by the preferential cis activity of the MOB Q4 relaxases. Such strategy would be biologically relevant in a scenario of co-residence of non-divergent elements to favor self-dissemination.