Abstract
A mutant F' plasmid has been isolated in a strain of Salmonella typhimurium harboring F(ts114)lac. This mutant, designated FlacS, exhibits unique genetic stability in strains of S. typhimurium and Escherichia coli. It shows no thermolability and is lost at frequencies of 20 to 100 times less than the wild-type F'lac (F42) in the same genetic backgrounds. The FlacS is also insensitive to conventional plasmid curing agents, whereas both F(ts114)lac and F42 are readily cured. The nature of the mutation(s) conferring stability to the FlacS is unclear, but plasmid linkage has been established. The high frequency of conjugal transfer of the FlacS and its behavior in recombination-deficient strains of S. typhimurium and E. coli argue against its stability being due to stable chromosomal integration. The FlacS is also capable of transferring chromosomal markers in S. typhimurium and E. coli mating systems. No major differences in chromosomal mobilization have been observed among F42, F(ts114)lac, and FlacS donors of either genus.