Stable inheritance of plasmid R1 requires two different loci.

The largest EcoRI fragment from plasmid R1 mediates a stability phenotype which is required to ensure the stable inheritance of this low-copy-number plasmid. When covalently linked to small, unstable R1 derivatives, this fragment makes the plasmids as stable as the wild-type R1 plasmid. A genetic analysis showed that two independently acting stabilization functions are encoded by this EcoRI fragment, both of which have the potential of partial stabilization of mini-R1 plasmids. The two loci are located at opposite ends of the fragment. Stabilization was also obtained by inserting these regions in unrelated, unstable plasmids from the p15 group. One of the two functions was very efficient in stabilizing such foreign replicons. Besides the stability phenotype, these genes exert incompatibility in an allele-specific manner. The stability functions do not seem to interfere seriously with the copy number of the plasmid.