The parVP region of the Salmonella typhimurium virulence plasmid pSLT contains four loci required for incompatibility and partition.

A 4332 nt MluI-PstI DNA fragment, earlier termed parVP, carries the incompatibility and partition determinants of the virulence plasmid, pSLT, of Salmonella typhimurium. This fragment was sequenced and the genes and regions responsible for incompatibility and partition phenotypes were identified by reference to previously published work. Incompatibility with pSLT, mediated by the 4332 nt fragment, requires both a homolog of the parS region of the Escherichia coli plasmids P1/P7 and a new incompatibility determinant, termed incR. The two genes identified in the sequence were also close homologs of the parA and parB loci of P1/P7. Unlike the situation in P1/P7, however, the parA and parB genes appear to be independently transcribed. Effective partition of pJRD158-based plasmids carrying fragments of the sequenced DNA required ParA and the incR region; ParB and parS were not necessary. It appears that pSLT may possess two related partitioning mechanisms, an analog of the recognized ParA/ParB/parS system and another system which requires ParA and incR.