Molecular cloning and analysis of the incompatibility and partition functions of the virulence plasmid of Salmonella typhimurium.

The incompatibility functions (inc) of the virulence plasmid of Salmonella typhimurium LT2 were initially located in a 4.3 kb region near the repA locus of the plasmid. Expression of inc required the presence, in cis or in trans, of two distinct DNA regions of the fragment. These regions, maximally 0.3 kb and 0.6 kb in size, were separated in the fragment by c. 3.0 kb. This intervening DNA encoded two proteins, of Mr values 37 kDa and 40 kDa. The promoter for the 37 kDa protein lay in or near one of the inc regions (incL). No function was assigned to this protein, however, it may be the product of a rep gene. The 40 kDa protein may have a partition (par) function, and may bind to a centromeric site in or near the other inc region (incR). An inc+ par- derivative of the original plasmid clone was used in a simple method, not involving the use of curing agents/mutagens, to eliminate virulence plasmid DNA from Salmonella typhimurium, Salmonella dublin, Salmonella enteritidis, and Salmonella cholerae-suis. The par function served to stabilise pJRD158B-based plasmid greater than 10(6)-fold in Escherichia coli and the virulence plasmid-cured Salmonella strains.