Abstract
The parB locus of plasmid R1, comprising at most 580 base pairs of DNA, mediates efficient plasmid stabilization via postsegregational killing of plasmid free cells. The locus encodes two small genes, hok and sok. The hok gene product is a potent cell killing protein, the expression of which is regulated by the sok product, an anti-sense RNA complementary to the hok mRNA. The hok mRNA is extraordinarily stable, while the sok-RNA is rapidly degraded. The mechanism of postsegregational killing is explained by the differential decay of the hok and sok-RNA's: In newborn plasmid free cells the prolonged persistence of the hok mRNA leads to synthesis of the Hok protein, thus ensuring a rapid and selective killing of these cells. As predicted from this simple model, any unstably inherited plasmid should become stabilized by carrying the parB locus. This important prediction was confirmed by testing a variety of different replicons. Even plasmids replicating in as distantly related organisms as Eschericia coli, Serratia marsescense, and Pseudomonas putida were efficiently stabilized. The parB locus therefore constitutes a convenient and efficient plasmid stabilization casette, useful in many gram negative species.
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