Real-time visualisation of conjugation reveals the molecular strategy evolved by the conjugative F plasmid to ensure the sequential production of plasmid factors during establishment in the new host cell

DNA conjugation is a contact-dependent horizontal gene transfer mechanism responsible for disseminating drug resistance among bacterial species. Conjugation remains poorly characterised at the cellular scale, particularly regarding the reactions occurring after the plasmid enters the new host cell. Here, we use live-cell microscopy to visualise the intracellular dynamics of conjugation in real time. We reveal that the transfer of the plasmid in single-stranded DNA (ssDNA) form followed by its conversion into double-stranded DNA (dsDNA) are fast and efficient processes that occur with specific timing and subcellular localisation. Notably, the ss-to-dsDNA conversion is the critical step that governs the timing of plasmid-encoded protein production. The leading region that first enters the recipient cell carries single-stranded promoters that allow the early and transient synthesis of leading proteins immediately upon entry of the ssDNA plasmid. The subsequent ss-to-dsDNA conversion turns off leading gene expression and licences the expression of the other plasmid genes under the control of conventional double-stranded promoters. This elegant molecular strategy evolved by the conjugative plasmid allows for the timely production of factors sequentially involved in establishing, maintaining and disseminating the plasmid.