Mutations within the replicon of the IncN plasmid pCU1 that affect its Escherichia coli polA-independence but not its autonomous replication ability.

The minimal replicon of the incompatibility N group plasmid pCU1 is contained within a 2-kb DNA region of the plasmid. The ability of this region and of the deletion derivatives thereof, that are capable of autonomous maintenance, to direct polypeptide synthesis was examined. Two proteins of 27 and 5.5 kDa are encoded by the minimal replicon. Polypeptide chain-terminating mutations within the predicted open reading frame for the 27-kDa polypeptide abolished the synthesis of this polypeptide and also the Escherichia coli polA-independence phenotype of the pCU1 replicon. However, these mutations did not affect the autonomous replication ability of the pCU1 replicon in wild-type E. coli and the expression of incompatibility towards the parental plasmid.