CONSTITUTIVE GLUCOSE-INDUCED ACTIVATION OF THE RAS-CAMP PATHWAY AND ABERRANT STATIONARY-PHASE ENTRY ON A GLUCOSE-CONTAINING MEDIUM IN THE SACCHAROMYCES-CEREVISIAE GLUCOSE-REPRESSION MUTANT HEX2

Addition of glucose to cells of the yeast Saccharomyces cerevisiae growing on a nonfermentable carbon source triggers a rapid, transient increase in the cAMP level. The occurrence of this cAMP spike appears to be correlated inversely with the glucose-repression state of the cells. This was also observed for the hex2 mutant, which is deficient in glucose repression and which displayed the cAMP signal constitutively. When cells of the hex2 mutant were starved for nitrogen on a glucose-containing medium, they rapidly lost viability, similarly to mutants with overactivation of the Ras-adenylate cyclase pathway, Flow cytometry measurements showed that G(1) arrest of the hex2 mutant under such conditions was incomplete. Trehalose accumulation, a typical feature of cells entering the stationary phase G(0), was very short-lived in the hex2 mutant under the same conditions. These results are in agreement with the presence of continuous glucose-triggered activation of cAMP synthesis in hex2 cells on a glucose-containing nitrogen-starvation medium. In the course of these experiments a spontaneous suppressor mutant, shx (for suppressor of hex2), was isolated which survived nitrogen starvation on a glucose-containing medium much better than the hex2 strain. It also showed normal G(1) arrest and much longer accumulation of trehalose. The suppressor mutation also caused inability to grow on nonfermentable carbon sources and absence of invertase derepression, and it was epistatic to hex2 for these characteristics also. The isolation of this epistatic derepression mutation supports the idea that the defect in glucose repression of the hex2 mutant is the cause of its rapid loss of viability during nitrogen starvation on a glucose-containing medium. Substitution of glucose for glycerol partially abolished the rapid loss of viability in the hex2 mutant. These results suggest that the glucose-repressible character of the pathway involved in glucose-triggered activation of cAMP synthesis might have a physiological role in preventing overstimulation of cAMP synthesis and allowing proper entrance into the stationary-phase G(0) in a medium containing ample glucose but lacking another essential nutrient for growth. Such a situation might be quite common in the glucose-rich natural environment of S. cerevisiae.