To achieve inorganic phosphate (Pi) homeostasis, cells must be able to sense intracellular and extracellular Pi concentrations. In the Pi signaling (PHO) pathway in Saccharomyces cerevisiae, high Pi represses genes involved in Pi uptake (e.g., PHO84) and Pi utilization (PHO5); conversely, the cyclin-dependent kinase inhibitor Pho81 inhibits the activity of the Pho80-Pho85 cyclin-cyclin dependent kinase complex in low-Pi conditions, leading to induction of these genes. However, how yeast senses

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... i availability remains unresolved. To identify factors involved in Pi sensing upstream of the Pho81-Pho80-Pho85 complex, we generated and screened suppressor mutants of a Δpho84 strain that shows constitutive PHO5 expression. By a series of genetic tests, including dominance-recessiveness, complementation and tetrad analyses, three sef (suppressor of pho84 [pho eighty-four]) mutants (sef8, sef9 and sef10) were shown to contain a novel single mutation. The sef mutants suppressed the phenotype of constitutive PHO5 expression at the transcriptional level, but did not show restored Pi uptake capacity. An epistasishypostasis test revealed that the sef mutations were hypostatic to pho80 mutation, indicating that their gene products function upstream of the Pho81-Pho80-Pho85 complex in the PHO pathway. The sef mutations identified are associated with gene(s) that may be involved in the homeostasis of an intracellular Pi level-sensing mechanism in S. cerevisiae.