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

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 degrees C, respectively, depended not only on temperature, theta ( degrees 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 theta 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 theta, D, or both in this instance. However, when the gene expression efficiency, epsilon (U/ng plasmid), was plotted versus C(p), the effects of theta and D on the expression became indistinct.