Genetic evidence that control of F replication is negative

We have taken advantage of two situations in which the incompatibility barrier between F plasmids is overcome to show that wild-type genes controlling F copy number (cop+) are dominant in trans over mutant genes. The simplest interpretation of our findings is that the cop mutations have inactivated a repressor gene that controls F replication. Since the cop. mutations all map in a region that others have shown by sequence analysis to theoretically encode four proteins, a strong possibility exists that one of these proteins is the repressor.