Abstract
BackgroundInorganic phosphate is an essential nutrient required by organisms for growth. During phosphate starvation, Saccharomyces cerevisiae activates the phosphate signal transduction (PHO) pathway, leading to expression of the secreted acid phosphatase, PHO5. The fission yeast, Schizosaccharomyces pombe, regulates expression of the ScPHO5 homolog (pho1+) via a non-orthologous PHO pathway involving genetically identified positive (pho7+) and negative (csk1+) regulators. The genes induced by phosphate limitation and the molecular mechanism by which pho7+ and csk1+ function are unknown. Here we use a combination of molecular biology, expression microarrays, and chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-Seq) to characterize the role of pho7+ and csk1+ in the PHO response.ResultsWe define the set of genes that comprise the initial response to phosphate starvation in S. pombe. We identify a conserved PHO response that contains the ScPHO5 (pho1+), ScPHO84 (SPBC8E4.01c), and ScGIT1 (SPBC1271.09) orthologs. We identify members of the Pho7 regulon and characterize Pho7 binding in response to phosphate-limitation and Csk1 activity. We demonstrate that activation of pho1+ requires Pho7 binding to a UAS in the pho1+ promoter and that Csk1 repression does not regulate Pho7 enrichment. Further, we find that Pho7-dependent activation is not limited to phosphate-starvation, as additional environmental stress response pathways require pho7+ for maximal induction.ConclusionsWe provide a global analysis of the transcriptional response to phosphate limitation in S. pombe. Our results elucidate the conserved core regulon induced in response to phosphate starvation in this ascomycete distantly related to S. cerevisiae and provide a better understanding of flexibility in environmental stress response networks.
Highlights
Inorganic phosphate is an essential nutrient required by organisms for growth
There are two distinct temporal responses in the phosphate signal transduction (PHO) pathway in S. pombe: a fast response concerned with immediately harvesting inorganic phosphate from the environment and transporting it into the cell, and a slower one associated with a general stress response
Within the fast response we define a core PHO regulon comprised of the pho1+, SPBC8E4.01c, and SPBC1271.09 genes whose induction in response to phosphate starvation, and regulatory behavior, has been conserved between S. pombe and S. cerevisiae
Summary
Saccharomyces cerevisiae activates the phosphate signal transduction (PHO) pathway, leading to expression of the secreted acid phosphatase, PHO5. The genes induced by phosphate limitation and the molecular mechanism by which pho7+ and csk1+ function are unknown. Inorganic phosphate (Pi) is an essential nutrient required for signal transduction, energy metabolism, and biochemistry in all organisms. Because external concentrations of inorganic phosphate fluctuate unpredictably, The transcription factors Pho and Pho play a key role in the phosphate starvation response in S. cerevisiae. Pho function can be conveniently monitored by measuring the activity of the secreted acid phosphatase, Pho, which is one of the most highly induced members of the PHO response [19,20]. The genes that comprise the PHO regulon have been well characterized and the precise sites within the genome where Pho binds during phosphate starvation are known [7,21]. Pho regulation occurs in response to changes in external phosphate levels and Pho activity is not thought to be regulated by other stress responses
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