Study on growth kinetics of unstable recombinant Saccharomyces cerevisiae

A mathematical model is suggested to describe effects of intracellular inhibition on growth of the recombinant yeast cells. The inhibition of the growth of the Leu+ phenotype cells depends upon the increase of plasmid concentration, the copy numbers and the accumulated amount of hepatitis B surface antigen protein, the expressed gene product. Simulating the growth of plasmid-free cells gives possible age-time distribution corresponding to their formation. According to the growth trajectories of these two different phenotype cells, a relation between Leu- phenotype cell growth and recombinant plasmid loss frequency is obtained. Consequently the instability of recombinant plasmid appearing in fermentation by genetically engineered S. cerevisiae is affected mainly due to (1) regular partitioning of Leu- phenotype combined with asymmetric plasmid segregation, and (2) different growth rates of plasmid-containing and plasmid-free cells in complex medium.