Effects of temperature and dilution rate on the copy number of recombinant plasmid in continuous culture ofBacillus stearothermophilus (pLP11)

Abstract

The penicillinase yield Y_P/X (U/g cell) of a transformant Bacillus stearothermophilus CU21 (pLP11) in continuous culture of LGP broth at 44, 47.5, and 50°C, respectively, depended not only on temperature, θ (°C), but also on dilution rate, D (h⁻¹), in a peculiar fashion that could not have been realized if the product were from the gene on the chromosome rather than the plasmid. The gene dosage effect could account for the unusual dependence of Y_P/X on θ and D, because the mode of Y_P/X as a function of D at a given temperature resembled that of plasmid content C_p (mg plasmid/g cell) vs. D at the same temperature. In other words, C_p, corresponding to the copy number of plasmid per grams of cell, could be controlled by either θ, D, or both in this instance. However, when the gene expression efficiency, ε (U/ng plasmid), was plotted versus C_p, the effects of θ and D on the expression became indistinct.