<i>GCN4</i> Encodes a Novel Transcription Factor in the UPR

<div><p>(A) Overexpression of <i>GCN4</i> is sufficient for activation of UPRE-2, but not UPRE-1 or UPRE-3. UPRE-driven transcriptional activity as a function of Gcn4p levels, elevated either as a result of overexpression (+ <i>GCN4–2μ</i>) or amino acid starvation (+ 3-AT), in the presence or absence of ER stress (Tm).</p> <p>(B) <i>GCN4</i> and <i>GCN2</i> are necessary for ER stress-dependent activation of UPRE-1 and UPRE-2. UPRE-driven transcriptional activity as a function of <i>GCN4</i> pathway genes (WT, Δ<i>gcn4,</i> and Δ<i>gcn2</i>) in the presence or absence of ER stress (Tm).</p> <p>(C) <i>GCN4</i> and <i>GCN2</i> are required for UPR-dependent transcriptional activation of a subset of target genes. Fold changes in mRNA levels were determined by microarray for DTT-treated vs. -untreated WT, Δ<i>ire1,</i> Δ<i>gcn4,</i> and Δ<i>gcn2</i> strains (columns). Histograms show distribution of log₂-fold changes for non-UPR target genes (light bars) and for UPR target genes (dark bars), which contain UPRE-1, UPRE-2, UPRE-3, or still unidentified UPREs (rows) in their promoters.</p> <p>(D) Target gene regulation differs significantly in WT and <i>Δgcn4/Δgcn2</i> mutants. Means <i>(μ)</i> and standard deviations <i>(σ)</i> for log₂-fold change in gene expression for non-UPR target genes, and for genes that fall inside the UPR target gene set and contain UPRE-1, UPRE-2, or UPRE-3 in their promoters. Z statistic <i>(z)</i> and <i>P</i> value <i>(P)</i>: higher <i>z</i> reflects a greater difference between the distribution for UPRE-containing target genes and nontarget genes; lower <i>P</i> indicates a more highly significant difference. For detailed calculations, see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0020246#s4" target="_blank">Materials and Methods</a>.</p>