Plasmid pKYM is a multicopy plasmid isolated from Shigella sonnei. We have determined the region required for the incompatibility which also controls the copy number. It is partially overlapped with the region required for the initiation of replication. There are two promoters in this region. One promoter synthesizes messenger ribonucleic acid (mRNA) of rep protein (PR promoter) and another one transcribes the opposite strand in the same region (PL promoter). These two RNAs seem to interact through the stem-and-loop structure present in this region. The repression of the synthesis of rep protein which is essential for plasmid replication by the interaction of these two RNAs seems to be the main mechanism of the incompatibility and the regulation of copy number.
