Plasmid instability kinetics of the yeast S288C pUCKm8 [cir+] in non-selective and selective media.

Plasmid loss kinetics for Saccharomyces cerevisiae transformed with the 2-mum DNA-based-plasmid pUCKm8 were measured in nonselective and selective media. The plasmid pUCKm8 gives the organism two new phenotypes: resistance to the wide spectrum antibiotic G418 sulfate, and the ability to produce the enzyme, beta-lactamase. Plasmid stability was determined using the production of beta-lactamase as a marker. The effect of G418 on the growth rates of all organisms present in the culture and on plasmid stability was also determined. Mathematical models describing plasmid loss kinetics during exponential growth for both nonselective and selective conditions are used to simulate the experimental data. In nonselective medium, over 80% of the cells still exhibited the desired phenotype after 50 doublings. In medium containing G418, improvements in plasmid stability were only marginal due to the appearance of antibiotic-resistant cells.