Hypothesis: hypersensitive plasmid copy number control for ColE1.

Initiation of replication of the plasmid ColE1 is primed by the cis-acting RNA II. Copy numbers are regulated by inhibition of RNA II by the antisense RNA I, whose concentration is proportional to the plasmid concentration. This inhibition is enhanced by a protein. Rom, and takes place during a time set by the transcription of 250 bases of the gene for RNA II. When this transcription is dominated by several steps of about equal duration, the probability for RNA II to prime DNA replication is approximately determined by e-constant[RNA I]. For large values of the "constant" small changes in [RNA I] give large variations in the priming probability. It is shown, first, that this type of mechanism can reduce the rate of plasmid loss and enable single copies of ColE1 to duplicate at a well-defined time in the cell cycle; second, that when the rate of initiation of transcription of RNA II increases, plasmid losses decrease and the distribution of single copy duplication times becomes narrower; third, that the action of Rom may further reduce plasmid losses and further narrow the distribution of duplication times in the single-copy case.