A model for copy number control of the plasmid R1.

A new model for copy number control of the plasmid R1 has been developed. It takes into account that initiation of replication of R1 requires a large number of cis-acting proteins (RepA). The theory explains how plasmid production rates respond to shifts in external conditions. It predicts the observed "eclipse" times between two plasmid duplications as well as the replication time for "runaway" plasmids lacking the antisense inhibitor CopA. The model also describes how the use of many cis-acting RepAs can lead to a tight coupling between cell and plasmid cycles that minimizes the rate of the plasmid loss. The results may be used as a guideline for construction of low copy number plasmids with high maintenance stability.