Control of replication of bacterial plasmids: genetics, molecular biology, and physiology of the plasmid R1 system.

Plasmids are autonomously replicating DNA molecules that are present in defined copy numbers in bacteria. This number may for some plasmids be very low (2-5 per average cell). In order to be stably inherited, replication and partitioning of the plasmid have to be strictly controlled. Plasmids carry genetic information for both processes. In the present paper we summarize what is known about the replication control system of one low-copy-number plasmid, R1, belonging to the FII incompatibility group. We do so because the FII group seems to be one of the best understood examples with respect to genetics, molecular biology, and physiology of the replication control system. The paper is not a classical review, but rather an essay in which we discuss the aspects of replication control that we regard as being important.