Effects of plasmid amplification and recombinant gene expression on the growth kinetics of recombinant E. coli

An experimental study was undertaken to identify and quantitate the effects of plasmid amplification and recombinant gene expression on Escherichia coli growth kinetics. Identification of these effects was possible because recombinant gene expression and plasmid copy number were controlled by different mechanisms on plasmid pVH106/172. Recombinant gene expression of the lactose operon structural genes was under the control of the lac promoter and was activated by the addition of the chemicals, IPTG and cyclic AMP, to the fermentation medium. Plasmid content was amplified in a separate fermentation by increasing culture temperature since the plasmid replicon was temperature-sensitive. A final fermentation was performed in which both plasmid content and recombinant gene expression were induced simultaneously by adding chemicals and raising the culture temperature. Recombinant growth rates were found to be reduced by the expression of high levels of recombinant lac proteins in the chemical induction experiments and by the amplification of plasmid levels in the temperature induction experiment. High expression of recombinant lac proteins following chemical induction was accompanied by a loss in recombinant cell viability. In the plasmid amplification experiment, the recombinant cells did not lose viability but the recombinant product yields were much lower than those achieved in the chemical induction experiments. Combining temperature and chemical induction increased the recombinant product yield by a factor of 4400 but also lowered cellular growth rates by 70%.