The mcr-1-bearing IncI2 plasmid promotes persistence of MDR IncFII plasmids through PcnR-mediated cross-plasmid copy-number regulation

Plasmid coexistence is widespread in multidrug-resistant Escherichia coli, yet the mechanisms maintaining co-resident resistance plasmids remain unclear. Here, we show that an mcr-1-bearing IncI2 plasmid promotes persistence of a co-resident MDR IncFII plasmid by coordinating plasmid replication and physiology. Genomic analysis revealed recurrent co-detection of IncI2 and IncFII plasmids, whereas conjugation assays showed that this pattern was not explained by enhanced basal transfer. Instead, IncI2 plasmid increased IncFII stability under antibiotic-free conditions and was associated with elevated intracellular ATP levels, long-term competitive advantage, and reduced IncFII copy number. Mechanistically, we identified a conserved antisense RNA-like element, repA tap, and demonstrated that the IncI2-encoded ProQ/FinO protein PcnR binds repA tap, promotes its interaction with repA_IncFII mRNA, and represses RepA synthesis, thereby reducing IncFII copy number. Furthermore, mcr-1 was associated with increased pcnR expression. Together, these findings identify a previously unrecognized cross-plasmid regulatory mechanism that promotes persistence of high-risk resistance plasmid combinations. Antibiotic resistance can be easily spread through plasmids, yet how bacteria can maintain multiple plasmids inside one cell remains unclear. The study shows that an mcr-1-bearing IncI2 plasmid promotes multidrug resistant IncFII persistence, while mcr-1 modulates PcnR-mediated IncFII copy-number regulation.