Activation of Saccharomyces cerevisiae trehalase by heat shock was shown in all strains tested, including mutants in which the response to a glucose signal was absent. A low concentration of cAMP favored the response as seen in 2nd log cells or in ras2 and cyr1ts mutant strains. The heat shock effect upon trehalase activity was not observed under conditions of catabolite repression. 2. Neither hexokinase PII nor the heat shock protein hsp26 seemed to be involved in the activation of trehalase by heat shock. However, mutant strains deleted in the polyubiquitin gene showed only a 2-fold activation of the enzyme while in control strains a 5- to 7-fold irreversible activation was observed. 3. An alternative mechanism of trehalase activation by removal of an inhibitor through ligation with ubiquitin is discussed. Activation by cAMP-independent phosphorylation is also considered.
