Characterization of a plasmid mutation affecting maintenance, transfer and elimination by novobiocin.

We have isolated a mutant H group plasmid temperature-sensitive for plasmid maintenance. Unlike the wild type plasmid (pSD114), the mutant (pDT4) was eliminated at 37 degrees C and also at 30 degrees C after novobiocin treatment. The mutant plasmid interfered with host cell growth at the non-permissive temperature. Conjugative transfer of the mutant was reduced at 30 degrees C compared to the wild-type plasmid. Introduction of a coumermycin-novobiocin resistance DNA gyrase (cou) mutation into Escherichia coli prevented pDT4 elimination by novobiocin but did not affect the temperature-sensitive phenotype. The evidence indicates that the mutant plasmid used bacterial DNA gyrase for replication. Models to account for the behaviour of this unusual mutant are discussed.