Copy number and the stability of 2-μm circle-based artificial plasmids of Saccharomyces cerevisiae

The copy number and stability of artificial 2-μm circle-based plasmids have been accurately measured in [Cir⁺] and [Cir⁰] strains of Saccharomyces cerevisiae. We conclude that (i) instability and copy number vary greatly from plasmid to plasmid; (ii) instability and copy number are negatively correlated - that is, high copy number is associated with low instability; (iii) it is difficult to reconcile this variability with a strict and direct system of copy number control; (iv) instabilities are much higher than expected from random partition and the observed copy numbers: this may imply partition which is less efficient than random. Even so, (v) the partitioning of 2-μm circle-like plasmids is more efficient than that of ARS-based plasmids, which hints at the existence of a system for the (inefficient) distribution of 2-μm circles.