IncC helper dependent plasmid-like replication of Salmonella Genomic Island 1

The Salmonella genomic island 1 (SGI1) and its variants are mobilized by IncA and IncC conjugative plasmids. SGI1-family elements and their helper plasmids are effective transporters of multidrug resistance determinants. SGI1 exploits the transfer apparatus of the helper plasmid and hijacks its activator complex, AcaCD, to trigger the expression of several SGI1 genes. In this way, SGI1 times its excision from the chromosome to the helper entry and expresses mating pore components that enhance SGI1 transfer. The SGI1-encoded T4SS components and the FlhDC-family activator proved to be interchangeable with their IncC-encoded homologs, indicating multiple interactions between SGI1 and its helpers. As a new aspect of this crosstalk, we report here the helper-induced replication of SGI1, which requires both activators, AcaCD and FlhDCSGI1, and significantly increases the stability of SGI1 when coexists with the helper plasmid. We have identified the oriVSGI1 and shown that S004-repA operon encodes for a translationally coupled leader protein and an IncN2/N3-related RepA that are expressed under the control of the AcaCD-responsive promoter PS004. This replicon transiently maintains SGI1 as a 4-8-copy plasmid, not only stabilizing the island but also contributing to the fast displacement of the helper plasmid.