Little is known about the potential interactions between the integrative elements that are found in bacterial chromosomes and plasmids. However, interactions between the integrative mobilisable element SGI1 (Salmonella genomic island 1) and IncC (or IncA) plasmids that mobilise SGI1 have been noted. We have previously shown that IncC plasmids prevent the conjugative transfer of another IncC plasmid into the cell where they reside. Complete exclusion was also seen between IncA and IncC plasmids and an entry exclusion determinant, eexC, was identified. To further understand the interactions between SGI1 and IncC (or IncA) plasmids, exclusion between the two elements was investigated using natural variants of SGI1.
When an IncC (or IncA) plasmid was present in the recipient, mobilisation of SGI1 from E. coli containing an SGI1 variant and a different IncC plasmid was unaffected but the plasmid was completely excluded. This indicates that the IncC plasmid could not exclude SGI1. However, when the IncC plasmid in the recipient was replaced by only eexC, exclusion of the IncC plasmid was no longer seen. Therefore, when SGI1 is in the donor, other gene(s) may be involved in exclusion of the IncC plasmid.
Whether SGI1 and its variants can exclude IncC and IncA plasmids was also investigated. When an SGI1 variant was present in recipient E. coli, IncC or IncA plasmid transfer was reduced ~15-240-fold relative to transfer into an isogenic recipient lacking SGI1. The region of SGI1 involved in exclusion of IncC plasmids has been narrowed to an ~6 kb region. Investigation of several open reading frames in this region has not yet identified the gene(s) involved. The observation that SGI1 can exclude IncC plasmids but IncC plasmids cannot exclude SGI1 provides further information about the types of interactions that can occur between integrative elements and plasmids.