Defect in the GTPase activating protein (GAP) function of eIF5 causes repression of GCN4 translation.

In eukaryotes, the eIF5 protein plays an important role in translation start site selection by providing the GAP (GTPase activating protein) function. However, in yeast translation initiation fidelity defective eIF5(G31R) mutant causes preferential utilization of UUG as initiation codon and is termed as Suppressor of initiation codon (Sui(-)) phenotype due to its hyper GTPase activity. The eIF5(G31R) mutant dominantly represses GCN4 expression and confers sensitivity to 3-Amino-1,2,4-Trizole (3AT) induced starvation. The down-regulation of the GCN4 expression (Gcn(-) phenotype) in the eIF5(G31R) mutant was not because of leaky scanning defects; rather was due to the utilization of upUUG initiation codons at the 5' regulatory region present between uORF1 and the main GCN4 ORF.

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