A mutation that decreases the efficiency of plasmid R1 replication leads to the activation of parD, a killer stability system of the plasmid.

The silent parD (kis/kid) stability operon of plasmid R1 is normally repressed by the co-ordinated action of the Kis and Kid proteins. In this report it is shown that a mutation in repA, the gene of the plasmid replication protein, that reduces two-fold the copy number of the plasmid, leads to the derepression of the parD system. This derepression can be prevented by a suppressor mutation in copB, a copy number control gene of plasmid R1, that increases the efficiency of replication of the repA mutant. Derepression of the wild-type parD system leads to high plasmid stability. These data show the activation of a plasmid stability operon by a mutation that reduces the efficiency of wild-type plasmid replication.