EFFECTS OF GROWTH ENVIRONMENT ON RECOMBINANT PLASMID STABILITY IN SACCHAROMYCES-CEREVISIAE GROWN IN CONTINUOUS-CULTURE

A recombinant strain of Saccharomyces cerevisiae, containing a 2-mu m- fragment-based plasmid (pYE alpha a4) was grown under non-selective co nditions in continuous culture. The decrease in the population carryin g the plasmid-encoded auxotrophic marker, LEU2, was examined under dif ferent 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-d ominated in cultures grown on defined media whereas Delta mu and R wer e co-dominant in cultures grown on complex medium. Loss rates increase d with increasing dilution rate in complex glucose-limited cultures. T he reverse was found in defined glucose-limited cultures. Plasmid rete ntion and loss kinetics determined from defined magnesium-limited cult ures were not significantly different from those observed in defined g lucose-limited cultures. Although plasmid retention in defined phospha te-limited culture was not significantly different from that in define d glucose-limited culture, reduced R and increased Delta mu indicated an alternative physiological effect of phosphate limitation on plasmid stability.