Role of RNA transcripts in replication incompatibility and copy number control in antibiotic resistance plasmid derivatives.

The genes required for autonomous replication and incompatibility in the antibiotic resistance plasmids R100 and R1 have been located within a 2.5-kilobase region of the 90-kilobase genome, within which the incompatibility gene occupies a 1.3-kilobase region excluding the replication origin. We now report that three RNA species are synthesized in vitro from the 2.5-kilobase region, which R100 and R1 have in common. One, a long RNA molecule which is transcribed in the direction of DNA replication, probably acts as a messenger or a protein required for plasmid replication. The second RNA species, only 91 nucleotides long, is transcribed in the opposite direction, from a region of the DNA entirely contained within the first and known to specify incompatibility and copy control functions. The third RNA species, 150 bases long, is transcribed from a region including the replication origin; it may be a primer of DNA synthesis or, in conjunction with the second of the three RNA species, an influence in the control of replication.