Conjugative transfer inhibition of IncA and IncC plasmids by 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 mobilize SGI1 at very high frequencies and are inhibited by SGI1. Here, we show that SGI1 and kin encode a fertility inhibitor, which we named CtiC (for 'Conjugative Transfer Inhibition of IncC'), that hampers A/C plasmid transfer. Fertility inhibition facilitates the propagation of SGI1, which is known to be destabilized by coresident A/C plasmids. ctiC expression is both constitutive and activated during conjugative transfer of A/C plasmids. Structural predictions suggest 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 and the C-terminus of TraI interact with the plasmid-encoded mobilization factor MobI, which recognizes and binds the origin of transfer (oriT) of A/C plasmids. Hence, CtiC acts as a decoy that competes with TraI for binding to MobI, freeing TraI for efficient SGI1 mobilization, and precluding efficient processing of the plasmid's oriT. Our work reveals a mechanism of fertility inhibition that prevents relaxosome assembly and uncovers a functional domain in MOBH12 relaxases.