[Analysis of chromosomal and extrachromosomal functions involved in the replication of bacterial plasmids]

A variety of functions codified by both the host chromosome and the plasmid genome are necessary for plasmid replication. We have found that dna B and dna F are essential for replication of both types of plasmids: R144 and Sa which have a restricted control of replication, and R6K with a relaxed control; on the other hand plasmids belonging to the first group absolutely depend on dna A and dna E gene products. Plasmid Sa, which shares with the 1st group a restricted control of replication but appears on a number of intermediate copies, shows a partial requirement to the dna A gene product. Location studies of plasmid-coded genes, involved in the replications of plasmids, were carried out in plasmid pSC102, a derivative of the resistance plasmic R6-5. All the genes required for replication are contained into a DNA segment which consists of two Pst I fragments: P-4 and P-6. The origin of replication On V is not itself sufficient for autonomous replication but needs in addition the presence of the Rep A gene. Genes responsible for copy control and incompatibility Cop/Inc are located on fragment P-6, i.e. they are physically distinct from Ori V. This indicates that the origin of replication is not responsible for, although may participate in, the incompatibility phenomenon.