The study explores how upstream open reading frames (uORFs) regulate ATF4 mRNA translation in mammalian cells. ATF4 is a key transcription factor involved in the stress response, and its expression is controlled by the differential contribution of two uORFs in the 5' leader of the mouse ATF4 mRNA. The 5' proximal uORF1 is a positive element that facilitates ribosome scanning and reinitiation at downstream coding regions in the ATF4 mRNA. When eIF2-GTP is abundant in nonstressed cells, ribosomes scanning downstream of uORF1 reinitiate at the next coding region, uORF2, an inhibitory element that blocks ATF4 expression. During stress conditions, phosphorylation of eIF2 and the accompanying reduction in eIF2-GTP levels increase the time required for scanning ribosomes to become competent to reinitiate translation. This delayed reinitiation allows ribosomes to scan through the inhibitory uORF2 and instead reinitiate at the ATF4-coding region, increasing ATF4 expression. This mechanism is conserved from yeast to mammals. The study also shows that the molecular mechanisms by which selected mRNAs are translated in response to eIF2 phosphorylation are similar in yeast and mammals. In yeast, the GCN4 mRNA contains four uORFs that regulate translation. The 40S ribosome bound to eIF2-GTP·Met-tRNA scans the mRNA, initiating translation at the 5'-proximal uORF1. After synthesis of the uORF1-encoded polypeptide, ribosomes reinitiate translation at a downstream coding region. The timing of reinitiation depends on the availability of eIF2-GTP. When eIF2-GTP is readily available, ribosomes reinitiate at inhibitory uORFs 2–4. However, when eIF2-GTP levels are reduced due to eIF2 phosphorylation, there is a delay in reinitiation that allows ribosomes to bypass the inhibitory uORFs and translate the GCN4-coding region. In mammals, the ATF4 mRNA also contains two uORFs. uORF1 is a positive element that facilitates translation of the ATF4-coding region in response to stress-induced eIF2 phosphorylation. By contrast, uORF2 is inhibitory, blocking ATF4 expression in nonstressed cells. These results suggest that higher eukaryotes have mechanisms of gene-specific translation control that share the hallmark features described for yeast GCN4. The study also shows that the presence of stem-loop structures or insert sequences in the ATF4 leader decreases its expression. The distance between uORF1 and uORF2 is optimized to allow scanning ribosomes to differentially reinitiate depending on the levels of available eIF2-GTP. The findings support the model that the primary role of uORF1 is toThe study explores how upstream open reading frames (uORFs) regulate ATF4 mRNA translation in mammalian cells. ATF4 is a key transcription factor involved in the stress response, and its expression is controlled by the differential contribution of two uORFs in the 5' leader of the mouse ATF4 mRNA. The 5' proximal uORF1 is a positive element that facilitates ribosome scanning and reinitiation at downstream coding regions in the ATF4 mRNA. When eIF2-GTP is abundant in nonstressed cells, ribosomes scanning downstream of uORF1 reinitiate at the next coding region, uORF2, an inhibitory element that blocks ATF4 expression. During stress conditions, phosphorylation of eIF2 and the accompanying reduction in eIF2-GTP levels increase the time required for scanning ribosomes to become competent to reinitiate translation. This delayed reinitiation allows ribosomes to scan through the inhibitory uORF2 and instead reinitiate at the ATF4-coding region, increasing ATF4 expression. This mechanism is conserved from yeast to mammals. The study also shows that the molecular mechanisms by which selected mRNAs are translated in response to eIF2 phosphorylation are similar in yeast and mammals. In yeast, the GCN4 mRNA contains four uORFs that regulate translation. The 40S ribosome bound to eIF2-GTP·Met-tRNA scans the mRNA, initiating translation at the 5'-proximal uORF1. After synthesis of the uORF1-encoded polypeptide, ribosomes reinitiate translation at a downstream coding region. The timing of reinitiation depends on the availability of eIF2-GTP. When eIF2-GTP is readily available, ribosomes reinitiate at inhibitory uORFs 2–4. However, when eIF2-GTP levels are reduced due to eIF2 phosphorylation, there is a delay in reinitiation that allows ribosomes to bypass the inhibitory uORFs and translate the GCN4-coding region. In mammals, the ATF4 mRNA also contains two uORFs. uORF1 is a positive element that facilitates translation of the ATF4-coding region in response to stress-induced eIF2 phosphorylation. By contrast, uORF2 is inhibitory, blocking ATF4 expression in nonstressed cells. These results suggest that higher eukaryotes have mechanisms of gene-specific translation control that share the hallmark features described for yeast GCN4. The study also shows that the presence of stem-loop structures or insert sequences in the ATF4 leader decreases its expression. The distance between uORF1 and uORF2 is optimized to allow scanning ribosomes to differentially reinitiate depending on the levels of available eIF2-GTP. The findings support the model that the primary role of uORF1 is to