Conjugal transfer of plasmid pMV158: uncoupling of the pMV158 origin of transfer from the mobilization gene mobM, and modulation of pMV158 transfer in Escherichia coli mediated by IncP plasmids.

The streptococcal plasmid pMV158 encodes a gene cassette involved in its mobilization by large conjugative plasmids. Two elements compose this region: i) the mobM gene, encoding the MobM protein that initiates transfer, and ii) the origin of transfer, oriT, which is the target of MobM. In vitro, MobM protein introduces a specific nick within the pMV158-oriT region on supercoiled pMV158 DNA. This paper reports the uncoupling of the oriT and the mobM gene, the latter being placed under the control of an inducible promoter. Upon induction, the vector containing pMV158-oriT was transferred in Escherichia coli matings at a moderate frequency whereas, in vitro, purified MobM protein efficiently cleaved the vector harbouring the pMV158-oriT. Transfer of this vector, as well as transfer of pMV158 in E. coli, required the presence of either the IncW R388 or the IncP RP4 plasmids as auxiliary plasmids. Dissection of the functions encoded by RP4 showed that the traG and traF genes were essential for pMV158 mobilization.