Gene-specific translational control of the yeast GCN4 gene by phosphorylation of eukaryotic initiation factor 2.

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2 alpha) is one of the best-characterized mechanisms for down-regulating total protein synthesis in mammalian cells in response to various stress conditions. Recent work indicates that regulation of the GCN4 gene of Saccharomyces cerevisiae by amino acid availability represents a gene-specific case of translational control by phosphorylation of eIF-2 alpha. Four short open reading frames in the leader of GCN4 mRNA (uORFs) restrict the flow of scanning ribosomes from the cap site to the GCN4 initiation codon. When amino acids are abundant, ribosomes translate the first uORF and reinitiate at one of the remaining uORFs in the leader, after which they dissociate from the mRNA. Under conditions of amino acid starvation, many ribosomes which have translated uORF1 fail to reinitiate at uORFs 2-4 and utilize the GCN4 start codon instead. Failure to reinitiate at uORFs 2-4 in starved cells results from a reduction in the GTP-bound form of eIF-2 that delivers charged initiator tRNA(iMet) to the ribosome. When the levels of eIF-2.GTP.Met-tRNA(iMet) ternary complexes are low, many ribosomes will not rebind this critical initiation factor following translation of uORF1 until after scanning past uORF4, but before reaching GCN4. Phosphorylation of eIF-2 by the protein kinase GCN2 decreases the concentration of eIF-2.GTP.Met-tRNA(iMet) complexes by inhibiting the guanine nucleotide exchange factor for eIF-2, which is the same mechanism utilized in mammalian cells to inhibit total protein synthesis by phosphorylation of eIF-2.