The formation of a dissociable plasmid cointegrate from the Flac factor and the resident plasmid of Salmonella typhimurium LT2.

The Flac factor showed unstable maintenance in Salmonella typhimurium dnaC MP10LT2. The properties of a more stable lac+ derivate (SD-1) are described. SD-1 was ts and carried the fi+ property and the ability to transfer the lac+ character. It contained a large plasmid of molecular weight about 129 X 10(6) DALTONS. The properties of SD-1 and its derivatives suggested that the large plasmid was a cointegate of Flac and the MP10lt2 plasmid. Lac+ transfer was efficient from SD-1 to M799 MP10LT2 and one lac+ exconjugant contained the intact cointegrate. The cointegrate was not successfully transferred to strains lacking MP10LT2. It dissociated into apparently unaltered Flac and MP10LT2 plasmids, but the deletion of small parts of one or both plasmids during cointegrate formation could not be ruled out. Cointegrate dissociation was more marked in M799 than in SD-1 especially during growth in glucose-Casamino acids minimal medium. In the presence of R1drd19, the cointegrate (like the MP10LT2 plasmid) was stable maintained in the dnaC strain; maintenance of Flac was, however, unstable. It seems likely that replication of the cointegrate was controlled by the MP10LT2 plasmid constitutent.