Incompatibility and transforming efficiency of ColE1 and related plasmids.

Replicons derived from the ColE1 plasmid are incompatible with one another, but are compatible with their naturally occurring relatives ColK and CloDF13. The incompatibility results in loss, by segregation, of one or the other ColE1 plasmid. In most cases, the smaller derivatives tend to displace the larger ones, and the rate of displacement depends on the difference in size. One mini-plasmid retains only 19% of the sequences of ColE1, yet it exerts strong incompatibility: other ColE1 plasmids are rapidly lost when it is introduced into the host. The region essential for ColE1 incompatibility is deduced to lie within 700 base pairs of the origin of replication. The transforming efficiency of any ColE1 plasmid is markedly lowered when another incompatible replicon is resident in the competent cells, even when the transforming plasmid is much smaller than the resident. A model of incompatibility is proposed to account for these effects.