An intracellular phosphate buffer filters transient fluctuations in extracellular phosphate levels.

To survive in a dynamic and unpredictable environment, cells must correctly interpret and integrate extracellular signals with internal factors. In particular, internal stores of nutrients must be managed for use during periods of nutrient limitation. To gain insight into this complex process, we combined biochemical and spectroscopic techniques to follow the dynamics of the phosphate responsive signaling pathway in both single yeast cells and populations. We demonstrate that the phosphate-responsive genes PHO5 and PHO84 exhibit different kinetics of transcriptional induction in response to phosphate starvation, and that transient phosphate limitation causes induction of PHO84 but not PHO5. This differential kinetic behavior is largely eliminated in cells that lack the ability to store phosphate internally in the form of polyphosphate, but the threshold of external phosphate required for induction of PHO5 and PHO84 is unaffected. Our observations indicate that polyphosphate acts as a buffer that can be mobilized during periods of phosphate limitation and enables the phosphate-responsive signaling pathway to filter transient fluctuations in extracellular phosphate levels.