Diverse mobilization strategies facilitate transfer of non-conjugative mobile genetic elements

- Non-conjugative mobile genetic elements are often mobilized by conjugative elements.
- Element carriage of a relaxase gene is not a strict requirement for mobilization.
- A relaxase-in trans mechanism likely mobilizes most staphylococcal plasmids.
- Mobilization mechanisms can be predicted for 92% of non-conjugative S. aureus plasmids.
- Diverse underappreciated mechanisms may mobilize most non-conjugative accessory elements.

Conjugation is a dominant mechanism of horizontal gene transfer and substantially contributes to the plasticity and evolvability of prokaryotic genomes. The impact of conjugation on genetic flux extends well beyond self-transmissible conjugative elements, because non-conjugative 'mobilizable elements' utilize other elements' conjugative apparatus for transfer. Bacterial genome comparisons highlight plasmids as vehicles for dissemination of pathogenesis and antimicrobial-resistance determinants, but for most non-conjugative plasmids, a mobilization mechanism is not apparent. Recently we discovered many Staphylococcus aureus plasmids lacking mobilization genes carry oriT sequences that mimic those on conjugative plasmids, suggesting that significantly more elements may be mobilizable than previously recognized. Here we summarize our findings, review the diverse mobilization strategies employed by mobile genetic elements and discuss implications for future gene-transfer research.

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