Description:
The RA3 plasmid, the archetype of IncU incompatibility group, represents a mosaic-modular genome of 45.9 kb. The replication module encompasses repA and repB (initiator) surrounded by two long repetitive sequences DR1 and DR2 of unknown function. Here, we mapped the origin of replication oriV to the 3′ end of repB and showed that oriV was activated by the transcription coming from orf02revp in the adjacent stability module. Using various in vivo and in vitro methods we demonstrated that the repB expression proceeded either from repBp located in the intergenic repA-repB region or from the upstream strong repAp that was autoregulated by RepA. Additionally, the repBp activity was modulated by the transcription from the overlapping, divergently oriented repXp. Both repXmRNA (antisense for repAmRNA) and its small polypeptide product, RepX, were strong incompatibility determinants. Hence, we showed that the sophisticated RA3 copy number control combined the multivalent regulation of repB expression, RepB titration by DR1, and transcriptional activation of oriV, dependent on the RA3 global regulatory network. Similarly organized replicons have been found in diverse bacterial species confirming the significance of these mechanisms in establishing the IncU plasmids in a broad spectrum of hosts.