Regulation of mini-F plasmid DNA replication. A quantitative model for control of plasmid mini-F replication in the bacterial cell division cycle.

A quantitative model for the regulation of replication of plasmid mini-F in the Escherichia coli cell division cycle has been developed. The essential repE gene of mini-F encodes a polypeptide that serves both as a positive replication initiation protein and as a regulatory repressor protein. The mini-F regulatory processes include the interaction of repressor with an operator site in the autogenous control of transcription of the repE gene, and the binding of initiator to repeated DNA sequences located both downstream from the repE gene and at the replication origin. A statistical thermodynamic model was used to predict probable configurations of the regulatory processes in a single growing cell. These probabilities were coupled by a kinetic model to events of the cell cycle such as mRNA transcription and protein translation, and the initiation of plasmid DNA replication. Parameter values were chosen so that the simulated values for plasmid copy number and repressor and initiator protein concentrations of the model agreed with experimentally determined estimates for mini-F. Simulations of deviations from regular segregation of plasmid copies at cell division and of premature or delayed initiation of plasmid replication suggest that mini-F replication control responds rapidly and precisely to these perturbations. The simulations also accurately mimic the response of plasmid mini-F to various plasmid copy number mutations and to various mini-F regulatory elements provided in trans. These simulations predict a stable pattern of inheritance for plasmid mini-F despite its low plasmid copy number, in agreement with experimental observation.