By cloning fragments of plasmid DNA, we have shown that RK2 expresses incompatibility by more than one mechanism. One previously identified (R. J. Meyer, Mol. Gen, Genet. 177:155--161, 1979; Thomas et al., Mol. Gen. Genet. 181:1--7, 1981) determinant for incompatibility is linked to the origin of plasmid DNA replication. When cloned into a plasmid vector, this determinant prevents the stable inheritance of a coresident RK2. However, susceptibility to this mechanism of incompatibility requires an active RK2 replicon and is abolished if another replicator is provided. We have also cloned a second incompatibility determinant, encoded within the 54.1- to 56.4-kilobase region of RK2 DNA, which we call IncP-1(II). An RK2 derivative remains sensitive to IncP-1(II), even when it is not replicating by means of the RK2 replicon. The 54.1- to 56.4-kilobase DNA does not confer susceptibility to the IncP-1(II) mechanism, nor does it encode a detectable system for efficient plasmid partitioning. The incompatibility may be related to the expression of genes mapping in the 54.1- to 56.4-kilobase region, which are required for plasmid maintenance and suppression of plasmid-encoded killing functions.
Discriminatory function of ribonuclease H in the selective initiation of plasmid DNA replication