Conjugative transfer inhibition of IncA and IncC plasmids by pervasive SGI1-like elements via relaxosome assembly interference

Broad-host-range IncA and IncC (A/C) conjugative plasmids propagate multidrug resistance in bacteria and mobilize chromosomal resistance islands, including Salmonella Genomic Island 1 (SGI1) across genera. A/C plasmids usually mobilize SGI1 at very high frequencies while being inhibited by SGI1. Here, we identified a broadly conserved fertility inhibitor, which we named CtiC (for Conjugative Transfer Inhibition of IncC), encoded by SGI1 and kin, that hampers A/C plasmid transfer. ctiC expression is both constitutive and activated during conjugative transfer of A/C plasmids. Our results indicate that the suppression of ctiC significantly enhances plasmid transfer, suggesting it counters a general improvement in conjugation mediated by the two genes traHG, located upstream of ctiC in SGI1. CtiC specifically prevents the cotransfer of the helper plasmid, which destabilizes SGI1 in transconjugants. Structural predictions revealed that CtiC resembles the C-terminus of the MOBH12-family relaxase TraI of A/C plasmids. Bacterial two-hybrid assays and relaxase domain substitutions show that CtiC interferes with relaxosome assembly by binding to the plasmid-encoded mobilization factor MobI, which recognizes the origin of transfer, thus preventing its interaction with TraI. Our work reveals a fertility inhibition mechanism that prevents relaxosome assembly and uncovers a functional domain in MOBH12 relaxases.