Figure 2. Long-term (molecular) and short-term (metabolic) responses in response to phosphate limitation.

(A) A schematic representation of a typical long-term cellular response upon severe phosphate starvation is shown (the S. cerevisiae PHO system is used illustratively). This response is mediated by gene expression changes through activation of phosphate responsive transcription factors (TFs). These transcription factors increase the expression of transcripts, which encode proteins involved in phosphate acquisition and release. These include phosphate transporters, acid- and alkaline-phosphatases (APases), and vacuolar phosphate transporters. These proteins result in both the increased uptake of extracellular phosphate and the release of phosphate from intracellular phosphate pools. (B) A schematic representation of a hypothetical set of biochemical pathways (a toy model) is shown; illustrating how these reactions can release or consume phosphates. In such a short-term metabolic response, which is elicited upon transient fluctuations in phosphate levels, changes in metabolic flux through specific nodes can immediately restore phosphate, either through increased synthesis of molecules that will release phosphate (highlighted in green box), or by decreased synthesis of molecules that will consume phosphate (highlighted in the red box).