The high-affinity cAMP phosphodiesterase of Saccharomyces cerevisiae is the major determinant of cAMP levels in stationary phase: involvement of different branches of the Ras-cyclic AMP pathway in stress responses.

The Ras-cyclic AMP (cAMP) pathway is a major determinant of intrinsic stress resistance of the yeast Saccharomyces cerevisiae. Here, we isolated IRA2, encoding the Ras GTPase activator, as a global stress response gene. Subsequently, we studied the other negative regulators on the separate branch of the Ras-cAMP pathway, the low- or high-affinity cAMP phosphodiesterase encoded by PDE1 or PDE2, respectively. Deletion of PDE2, similar to ira2 deletion, rendered cells sensitive to freeze-thawing, peroxides, paraquat, cycloheximide, heavy metals, NaCl, heat, or cold shock. However, deletion of PDE1 did not affect stress tolerance, although it exacerbated stress sensitivity caused by the pde2 deletion, indicating that PDE1 can partly compensate for PDE2. Deletion of IRA2 uniquely led to high sensitivity to cumene hydroperoxide, suggesting that IRA2 may have a distinct role for the response to this stress. Stress sensitivity of yeast cells in general correlated with the basal level of cAMP. Interestingly, yeast cells lacking PDE2 maintained higher cAMP levels in stationary phase than exponential growth phase, suggesting that Pde2p is the major regulator of cAMP levels in stationary phase. Depletion of Ras activity could not effectively suppress stress sensitivity caused by lack of cAMP phosphodiesterases although it could suppress stress sensitivity caused by lack of IRA2, indicating that cAMP accumulation in stationary phase can be mediated by other signaling proteins in addition to Ras. Our study shows that control of cAMP basal levels is important for determining intrinsic stress tolerance of yeast, and that the cAMP level during stationary phase is a result of a dynamic balance between its rates of synthesis and degradation.