Interaction between the RP4 coupling protein TraG and the pBHR1 mobilization protein Mob

It is currently believed that interaction between the relaxosome of a mobil izable plasmid and the transfer machinery of the helper conjugative plasmid is mediated by a TraG family coupling protein. The coupling proteins appea r as an essential determinant of mobilization specificity and efficiency. U sing a two-hybrid system, we demonstrated for the first time the direct in vivo interaction between the coupling protein of a conjugative plasmid (the TraG protein of RP4) and the relaxase of a mobilizable plasmid (the Mob pr otein of pBHR1, a derivative of the broad host range plasmid pBBR1). This i nteraction was confirmed in vitro by an overlay assay and was shown to occu r even in the absence of the transfer origin of pBHR1. We showed that, amon g 11 conjugative plasmids tested, pBHR1 is efficiently mobilized only by pl asmids encoding an IncP-type transfer system. We also showed that the RP4 T raG coupling protein is essential for mobilization of a pBBR1 derivative an d is the element that allows its mobilization by R388 plasmid (IncW) at a d etectable frequency.