Can SGI1 family integrative mobilizable elements overcome entry exclusion exerted by IncA and IncC plasmids on IncC plasmids?

Though IncC and IncA plasmids are compatible, they exert high level exclusion on one another. Here, the question of whether the presence of an SGI1 family element in the donor can overcome the exclusion of an IncC plasmid exerted by an IncC or IncA plasmid in the recipient was investigated. The transfer of the integrative mobilizable element SGI1 and its many variant forms into a new host is dependent on transfer machinery supplied by IncC or IncA plasmids. SGI1 elements include the determinants of a mobilization system and three genes that encode homologues of transfer proteins including TraG. Exclusion of a complete IncC plasmid by a complete IncA or IncC plasmid in the recipient was not ameliorated by an SGI1 element in the donor. However, transfer of the SGI was unaffected indicating that a functional mating apparatus was formed. The presence of only the plasmid-derived eexC or eexA gene in the recipient exerted high level exclusion on an incoming IncC plasmid and this was overcome by an SGI1 variant in the donor. Hence, the SGI affects only entry exclusion and additional plasmid features must influence other routes to plasmid exclusion.