Nutrient-Regulated Antisense and Intragenic RNAs Modulate a Signal Transduction Pathway in Yeast

Open Access
Publisher Website
Google Scholar
Abstract
The budding yeast Saccharomyces cerevisiae alters its gene expression profile in response to a change in nutrient availability. The PHO system is a well-studied case in the transcriptional regulation responding to nutritional changes in which a set of genes (PHO genes) is expressed to activate inorganic phosphate (Pi) metabolism for adaptation to Pi starvation. Pi starvation triggers an inhibition of Pho85 kinase, leading to migration of unphosphorylated Pho4 transcriptional activator into the nucleus and enabling expression of PHO genes. When Pi is sufficient, the Pho85 kinase phosphorylates Pho4, thereby excluding it from the nucleus and resulting in repression (i.e., lack of transcription) of PHO genes. The Pho85 kinase has a role in various cellular functions other than regulation of the PHO system in that Pho85 monitors whether environmental conditions are adequate for cell growth and represses inadequate (untimely) responses in these cellular processes. In contrast, Pho4 appears to activate some genes involved in stress response and is required for G1 arrest caused by DNA damage. These facts suggest the antagonistic function of these two players on a more general scale when yeast cells must cope with stress conditions. To explore general involvement of Pho4 in stress response, we tried to identify Pho4-dependent genes by a genome-wide mapping of Pho4 and Rpo21 binding (Rpo21 being the largest subunit of RNA polymerase II) using a yeast tiling array. In the course of this study, we found Pi- and Pho4-regulated intragenic and antisense RNAs that could modulate the Pi signal transduction pathway. Low-Pi signal is transmitted via certain inositol polyphosphate (IP) species (IP7) that are synthesized by Vip1 IP6 kinase. We have shown that Pho4 activates the transcription of antisense and intragenic RNAs in the KCS1 locus to down-regulate the Kcs1 activity, another IP6 kinase, by producing truncated Kcs1 protein via hybrid formation with the KCS1 mRNA and translation of the intragenic RNA, thereby enabling Vip1 to utilize more IP6 to synthesize IP7 functioning in low-Pi signaling. Because Kcs1 also can phosphorylate these IP7 species to synthesize IP8, reduction in Kcs1 activity can ensure accumulation of the IP7 species, leading to further stimulation of low-Pi signaling (i.e., forming a positive feedback loop). We also report that genes apparently not involved in the PHO system are regulated by Pho4 either dependent upon or independent of the Pi conditions, and many of the latter genes are involved in stress response. In S. cerevisiae, a large-scale cDNA analysis and mapping of RNA polymerase II binding using a high-resolution tiling array have identified a large number of antisense RNA species whose functions are yet to be clarified. Here we have shown that nutrient-regulated antisense and intragenic RNAs as well as direct regulation of structural gene transcription function in the response to nutrient availability. Our findings also imply that Pho4 is present in the nucleus even under high-Pi conditions to activate or repress transcription, which challenges our current understanding of Pho4 regulation. How does a microorganism adapt to changes in its environment? Phosphate metabolism in the budding yeast Saccharomyces cerevisiae serves as a model for investigating mechanisms involved in physiological adaptation. The nutrient inorganic phosphate (Pi) is essential for building nucleic acids and phospholipids; when yeast cells are deprived of Pi, genes required for scavenging the nutrient are activated. This activation is mediated by the Pho4 transcription factor through its migration into or out of nucleus. The Pi-starvation (low-Pi) signal is transmitted by a class of inositol polyphosphate (IP) species, IP7, which is synthesized by one of two IP6 kinases, Vip1 or Kcs1. However, the IP7 made primarily by Vip1 is key in the signaling pathway. Here we report that under Pi starvation Pho4 binds within the coding sequence of KCS1 to activate transcription of both intragenic and antisense RNAs, resulting in the production of a truncated Kcs1 protein and the likely down-regulation of Kcs1 activity. Consequently Vip1 can produce more IP7 to enhance the low-Pi signaling and thus form a positive feedback loop. We have also demonstrated that Pho4 regulates, both positively and negatively, transcription of genes apparently uninvolved in cellular response to Pi starvation and that it sometimes does so independently of Pi conditions. These findings reveal mechanisms that go beyond the currently held model of Pho4 regulation.