Nathan FRAIKIN - Coupling DNA processing to early gene expression facilitates conjugative plasmid establishment

Conjugative plasmids are small circular DNA molecules that can be transferred from bacteria to bacteria and represent the main vector for antimicrobial resistance dissemination. Plasmids enter new recipient bacteria as single-stranded (ss) DNA and must be converted to double-stranded (ds)DNA to complete their life cycle. During this establishment phase, plasmids are vulnerable to host defences and transcribe a specific set of early, protective genes to facilitate this establishment, a phenomenon known as zygotic induction. Our model, the F plasmid, expresses a small subset of zygotic genes using ssDNA promoters. However, the F plasmid encodes several other protective anti-defence genes whose transcriptional regulation is not known. Our results identified two new classes of ssDNA promoters present on F as well as on other plasmid families and that transcribe genes primarily implicated in the neutralization of restriction-modification (RM) systems. Using a fluorescent reporter system to monitor ss-to-dsDNA conversion at various plasmid loci, we show that ssp2, one of these novel promoters, act as a replication roadblock during the ss-to-dsDNA conversion process. This replicative delay is temporary, alleviated by the UvrD helicase and extends the single-stranded state of zygotic genes and ssDNA promoters. In turn, this enhances early expression of protective zygotic genes and facilitates plasmid dissemination in non-isogenic hosts that harbour RM systems.