Effects of growth environment on recombinant plasmid stability in Saccharomyces cerevisiae grown in continuous culture.

A recombinant strain of Saccharomyces cerevisiae, containing a 2-microns-fragment-based plasmid (pYE alpha a4) was grown under non-selective conditions in continuous culture. The decrease in the population carrying the plasmid-encoded auxotrophic marker, LEU2, was examined under different physiological conditions. The difference in growth rate (delta mu) between plasmid-free and plasmid-containing cells and the rate of plasmid segregation (R) were determined using a non-linear regression technique. Loss rates were greater in defined glucose-limited cultures than in complex glucose-limited cultures. Plasmid loss was delta mu-dominated in cultures grown on defined media whereas delta mu and R were co-dominant in cultures grown on complex medium. Loss rates increased with increasing dilution rate in complex glucose-limited cultures. The reverse was found in defined glucose-limited cultures. Plasmid retention and loss kinetic determined from defined magnesium-limited cultures were not significantly different from those observed in defined glucose-limited cultures. Although plasmid retention in defined phosphate-limited culture was not significantly different from that in defined glucose-limited culture, reduced R and increased delta mu indicated an alternative physiological effect of phosphate limitation on plasmid stability.