bioreactor studies of heterologous protein production by ...

dilution rate exceeded a certain value and Hassan, 1988). These results on the
dilution rate effect are remarkably consistent considering the di&ruit yeast strains that
were tested. The effect of dilution rate seems to relate to the media used. In a
nonselective medium, increashg dilution rate was accompanied by increasing
segregational instability of plasmids; however, in a çelective medium, at high dilution rate
the fraction of plasmid-containing œiIs remained ~latively constant for about 100
generations (Hardjito et ai., 1993). impoolsup et ai. (1989) examined the stabiliv of 2 p
based yeast plasmid (pLG669-z) d u ~ g continuous culture with cyclic growth rate
changes in a nonselective medium, and showed that plasmid stability kIl at high dilution
rate. In their experiments, the dilution rate was cycled beween a very low growth rate
(0.075 h") and a higher growth rate (0.25 h"). During growth at the high dilution rate,
the fraction of plasmid-containhg ceiis and the expression feU After the dilution rate was
changed to a low dilution rate, the fraction and expression level rose ahost to the initial
values (Irnpoolsup et al., 1989). Higher stability of a yeast plasmid at lower dilution rate
in nonselective media was obsemd also by Cam et al. (1990). Attwnpts have been made
to expiain these effects theoreticaliy at the molecrilar and ceIIular levels, as weU as through
macroscopic models and simulations (Parker and DiBiasio, 1987; Hjortso and Bailey,
1984a; DiBiasio and Sardonini, 1986; Srienœ et ai., 1986; Sdrdonini and DiBiasio, 1987;
Stephens and Lyberatos, 1988). but acmai mechanisms remain unclear.
The degree of plasmid instabîüty can be detennined by the probability of plasmid
ioss (p) and the growth ratio (a) (Imanaka and 1981). The probability of plasnid