Maintenance and incompatibility of plasmids carrying the replication origin of the Escherichia coli chromosome: evidence for a control region of replication between oriC and asnA.

Plasmids that replicate only by means of the cloned Escherichia coli replication origin (oriC) are called minichromosomes or oriC-plasmids. In this paper it is shown that sequences located between oriC and asnA are involved in maintenance and incompatibility of minichromosomes. These sequences include part of the 16kD and 17kD genes, previously allocated within this region (1,2). Transcription towards oriC that is initiated at the 16kD promoter, specifically enhances the stability and copy-number of minichromosomes. Three regions are involved in minichromosome incompatibility. One region, incA, includes the minimal oriC sequence. A second, incB, maps within a 210 base pairs fragment that overlaps the 16kD promoter. The third, incC, encompasses the 17kD gene. Neither one of the regions expresses incompatibility on its own, but the additional presence of one of the others is required. The data presented indicate that sequences of the 16kD and 17kD genes are part of the replication control system of oriC-plasmids.