An effect of F-like plasmids on the maintenance of Flac in a dnaC mutant of Salmonella typhimurium.

Flac maintenance was aberrant at permissive temperature in a temperature-sensitive dnaC mutant of Salmonella typhimurium when the normally resident pLT2 plasmid was present. Flac was, however, efficiently transferred into the dnaC pLT2+ strain and the resulting Flac derivative was almost as efficient in transferring Flac as was dnaC+ pLT2+ Flac strains indicating that aberrant Flac maintenance was not associated with appreciable inhibition of transfer replication. A range of F-like plasmids behaved like pLT2 in causing aberrant Flac maintenance when present in the dnaC pLT2- strain. Flac was, however, stably maintained in the dnaC strain in the absence of other plasmids. Although the F-like plasmids destabilized Flac, each was stably maintained when introduced into strain 11G dnaC pLT2+ and pLT2 was also apparently stable under these conditions. The destabilizing effect of pLT2 and other fi+ plasmids was not consequent upon their inhibiting the formation of a repressible F transfer component needed for Flac replication in the dnaC strain. Incompatibility between Flac and the other plasmids induced by the dnaC lesion also appeared unlikely to be a cause of the aberrant Flac maintenance. The possibility is discussed that the initiation of Flac replication differs from that of pLT2 and the F-like plasmids with F competing less effectively than the others for the DnaC gene product.