Expression of β‐lactamase by recombinant Escherichia coli strains containing plasmids of different sizes—effects of pH, phosphate, and dissolved oxygen

The characteristics of growth and synthesis of plasmid‐encoded protein were studied for strains of recombinant E. coli JM103 which carried the β‐lactamase gene on plasmids of different sizes. The plasmids used included the vector pUC8 and its recombinant derivatives containing varying‐sized inserts of Drosophila DNA (not expressed in E. coli ). Luria broth (LB) and a minimal medium (M9) supplemented in some cases with additional inorganic phosphate were used as growth media. There was no evidence of segregational instability in these experiments, where no antibiotic selection pressure was employed. Responses of the recombinant strains to variations in environmental parameters including pH, phosphate concentration in the medium, and aeration rate were examined. While the cell growth rate in LB decreased with pH in the range 7.0–8.0, the bulk β‐lactamase activity was maximized at an intermediate pH. The recombinant cell growth rate decreases with increasing plasmid size in the minimal medium, while such decrease is not significant when a rich medium such as LB is used. There is an intermediate plasmid size in the range studied (2.7–8.7 kb), at which β‐lactamase activity is maximum. While reduction in aeration rate (which determines the dissolved oxygen level) is detrimental for cell growth, it is beneficial for β‐lactamase synthesis. The bulk β‐lactamase activity therefore exhibits a maximum with respect to aeration rate. Cell growth and β‐lactamase production are affected in a similar manner by phosphate concentration in the minimal medium and therefore both are maximized at the same phosphate concentration. This investigation demonstrates clearly how the production of a recombinant plasmid‐encoded protein can be maximized by proper manipulation of culture conditions and how it is affected by plasmid size.