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

Abstract

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.

Crucial role ofSalmonellagenomic island 1 master activator in the parasitism of IncC plasmids

“…In contrast, suppression of sgaDC of SGI1 in the presence of the IncC plasmid abolishes SGI1 replication and allows peaceful coexistence of both elements despite the presence of a fully functional copy of acaDC . During revision of this manuscript, others have confirmed the importance of SgaCD in SGI1 replication ( 32 ). Consistent with our results, SGI1-K that lacks sgaDC due to a 2779-bp deletion extending from S008 to the 5′ half of traN S ( S005 ) is compatible with IncC plasmids ( 35 , 42 ).…”

“…Contrasting with our observations with SGI1, deletion of sgaDC was shown to have no impact on the mobilisation of SGI1-C, whereas deletion of acaDC completely abolished both plasmid self-transfer and SGI1-C mobilisation ( 28 , 29 ). Moreover, excision and replication of SGI1-C are undetectable in the presence of the Δ acaDC mutant of the helper IncC plasmid ( 32 ). Hence, unlike sgaDC of SGI1, sgaDC of SGI1-C seems to be unable to complement an acaDC null mutant of its helper plasmid or is perhaps not even functional.…”

“…Consistent with our results, SGI1-K that lacks sgaDC due to a 2779-bp deletion extending from S008 to the 5′ half of traN S ( S005 ) is compatible with IncC plasmids ( 35 , 42 ). Interestingly, SGI1-C cannot excise and replicate in cells bearing a Δ acaDC helper IncC plasmid, and fails to complement the transfer of such a mutant ( 28 , 32 ). Furthermore, the sgaDC deletion has no impact on SGI1-C mobilisation, excision or replication in the presence of a wild-type IncC plasmid, suggesting that it is not functional.…”

“…rep encodes the replication initiator protein that initiates SGI1 replication at the origin of replication ( oriV ) ( 31 ). Rep contains a RepA_C domain (Pfam PF04796) and is distantly related to RepA of IncN2 plasmids ( 31 , 32 ). traN S , traH S and traG S encode three type IV secretion system (T4SS) subunits that replace their respective counterparts TraN C , TraH C and TraG C in the mating pore encoded by the helper IncC plasmid ( 33 ).…”

Genomic islands targeting dusA in Vibrio species are distantly related to Salmonella Genomic Island 1 and mobilizable by IncC conjugative plasmids

“…[26]. Multiple GIs also carry putative replicons based on repA and trfA (Fig 1), suggesting that independent replication is crucial in the lifecycle of these GIs, perhaps to improve their stability in the presence of their helper plasmid [26][27][28][29][30].…”