The role of the Yap5 transcription factor in remodeling gene expression in response to Fe bioavailability

Negative feedback regulation of the yeast CTH1 and CTH2 mRNA binding proteins is required for adaptation to iron deficiency and iron supplementation

Iron (Fe) is an essential element for all eukaryotic organisms because it functions as a cofactor in a wide range of biochemical processes. Cells have developed sophisticated mechanisms to tightly control Fe utilization in response to alterations in cellular demands and bioavailability. In response to Fe deficiency, the yeast Saccharomyces cerevisiae activates transcription of the CTH1 and CTH2 genes, which encode proteins that bind to AU-rich elements (AREs) within the 3' untranslated regions (3'UTRs) of many mRNAs, leading to metabolic reprogramming of Fe-dependent pathways and decreased Fe storage. The precise mechanisms underlying Cth1 and Cth2 function and regulation are incompletely understood. We report here that the Cth1 and Cth2 proteins specifically bind in vivo to AREs located at the 3'UTRs of their own transcripts in an auto- and cross-regulated mechanism that limits their expression. By mutagenesis of the AREs within the CTH2 transcript, we demonstrate that a Cth2 negative-feedback loop is required for the efficient decline in Cth2 protein levels observed upon a rapid rise in Fe availability. Importantly, Cth2 autoregulation is critical for the appropriate recovery of Fe-dependent processes and resumption of growth in response to a change from Fe deficiency to Fe supplementation.

A role for iron-sulfur clusters in the regulation of transcription factor Yap5-dependent high iron transcriptional responses in yeast

Yeast respond to increased cytosolic iron by activating the transcription factor Yap5 increasing transcription of CCC1, which encodes a vacuolar iron importer. Using a genetic screen to identify genes involved in Yap5 iron sensing, we discovered that a mutation in SSQ1, which encodes a mitochondrial chaperone involved in iron-sulfur cluster synthesis, prevented expression of Yap5 target genes. We demonstrated that mutation or reduced expression of other genes involved in mitochondrial iron-sulfur cluster synthesis (YFH1, ISU1) prevented induction of the Yap5 response. We took advantage of the iron-dependent catalytic activity of Pseudaminobacter salicylatoxidans gentisate 1,2-dioxygenase expressed in yeast to measure changes in cytosolic iron. We determined that reductions in iron-sulfur cluster synthesis did not affect the activity of cytosolic gentisate 1,2-dioxygenase. We show that loss of activity of the cytosolic iron-sulfur cluster assembly complex proteins or deletion of cytosolic glutaredoxins did not reduce expression of Yap5 target genes. These results suggest that the high iron transcriptional response, as well as the low iron transcriptional response, senses iron-sulfur clusters.