Catabolite inactivation of the galactose uptake system in yeast.

In order to investigate the mechanism of inactivation of the galactose-fermenting system (“galactozymase”) after addition of glucose to galactose-grown cells of Saccharomyces cereuisiae the activities of the following enzymes of galactose utilization were assayed: the galactose uptake system, galactokinase, galact,ose-l-phosphate uridyltransferase, UDP-galactose-4-epimerase, and phosphoglucomutase. After 4 h of incubation of the cells with glucose the apparent K,,, of the galactose uptake system increased from 3.6 to 11 mM galactose, whereas I/,,, remained constant at 3.9 pmol of galactose x mini-’ x g wet weight-.‘. Cycloheximide (6.25 pglml) had no influence on inactivation, that is on this increase of the apparent K,,, for galactose, but completely prevented recovery of the activity of the galactose uptake system on further incubation of the cells in a galactose-containing, glucose-free medium. In contrast to the observation on the galactose uptake system, no significant changes were observed in the activities of the enzymes of the pathway from intracellular galactose to glucose &phosphate (galactokinase, galactose-l-phosphate uridyltransferase, UDP-galactose-4-epimerase, and phosphoglucomutase) after addition of glucose to galactose-grown cells. From these findings and from the observation that the galactose uptake system exhibits the lowest specific activity of the five enzymes which channel galactose into the glucose degradation pathway, it is concluded that a decrease in the affinity for galactose of the galactose uptake system is responsible for the catabolite inactivation of the galactose-fermenting system (galactozymase) in S. cereuisiae.