Salmonella genomic island 1 (SGI1) reshapes the mating apparatus of IncC conjugative plasmids to promote self-propagation

IncC conjugative plasmids and Salmonella genomic island 1 (SGI1) and relatives are frequently associated with multidrug resistance of clinical isolates of pathogenic Enterobacteriaceae. SGI1 is specifically mobilized in trans by IncA and IncC plasmids (commonly referred to as A/C plasmids) following its excision from the chromosome, an event triggered by the transcriptional activator complex AcaCD encoded by these helper plasmids. Although SGI1 is not self-transmissible, it carries three genes, traN(S), traH(S) and traG(S), coding for distant homologs of the predicted mating pore subunits TraN(C), TraH(C) and TraG(C), respectively, encoded by A/C plasmids. Here we investigated the regulation of traN(S) and traHG(S) and the role of these three genes in the transmissibility of SGI1. Transcriptional fusion of the promoter sequences of traN(S) and traHG(S) to the reporter gene lacZ confirmed that expression of these genes is inducible by AcaCD. Mating experiments using combinations of deletion mutants of SGI1 and the helper IncC plasmid pVCR94 revealed complex interactions between these two mobile genetic elements. Whereas traN(C) and traHG(C) are essential for IncC plasmid transfer, SGI1 could rescue null mutants of each individual gene revealing that TraN(S), TraH(S) and TraG(S) are functional proteins. Complementation assays of individual tra(C) and tra(S) mutants showed that not only do TraN(S)/H(S)/G(S) replace TraN(C)/H(C)/G(C) in the mating pore encoded by IncC plasmids but also that traG(S) and traH(S) are both required for SGI1 optimal transfer. In fact, remodeling of the IncC-encoded mating pore by SGI1 was found to be essential to enhance transfer rate of SGI1 over the helper plasmid. Furthermore, traG(S) was found to be crucial to allow DNA transfer between cells bearing IncC helper plasmids, thereby suggesting that by remodeling the mating pore SGI1 disables an IncC-encoded entry exclusion mechanism. Hence tra(S) genes facilitate the invasion by SGI1 of cell populations bearing IncC plasmids.