Effect of growth rate and expression level on plasmid stability in Saccharomyces cerevisiae.

An auxotrophic mutant of Saccharomyces cerevisiae, containing a recombinant 2-micro-based plasmid, was grown in selective media in continuous culture. The plasmid retained the ability to synthesize acid phosphatase as product, which was deleted from the host. Plasmid loss was followed at various dilution rates, and the level of plasmid expression was controlled by changing the beta-glycero/inorganic phosphate ratio.Some interesting trends were observed. As the level of plasmid expression was raised, the stability dropped markedly. Since acid phosphatase expression is regulated at the level of transcription, it is possible that increased transcription interfered with plasmid replication, hindered segregation, or overburdened the cell's DNA repair capability. It was also observed that plasmid stability was substantially increased at high growth rates. At dilution rates of 0.3 and 0.37 h(-1), feeding only inorganic phosphate, the plasmid was completely stable.