Transient responses of yeast continuous cultures to qualitative changes in the nutrient supply. Induction and repression of the galactose pathway enzymes

Induction and repression kinetics of alphagalactosidase, galactose uptake system and Leloir pathway enzymes were studied in chemostat cultures by changing the medium feed from glucose (11 mM) to glucose and galactose (11 mM; 17 mM respectively) in the induction experiments; and from galactose (11 mM) or (111 mM) to galactose plus glucose (83 mM) in the repression experiments. Basal levels of alpha-galactosidase and glucose uptake could be estimated in glucose-limited yeast cells, but it was not possible to detect any glactose pathway enzyme activity. In the repression experiments under galactose-limited or galactose-sufficient yeast cells, alpha-galactosidase and galactokinase decayed with K_d=-0.21h^-1=-D; that is, synthesis of these enzymes ceased (catabolite repression). In contrast transferase and epimerase activities and galactose uptake, decreased with K_d values of-0.33 and-0.54h^-1, showing that these activities were also subject to catabolite inactivation.