Repression of GCN4 mRNA translation by nitrogen starvation in Saccharomyces cerevisiae

Saccharomyces cerevisiae activates a regulatory network called "general con trol" that provides the cell with sufficient amounts of protein precursors during amino acid starvation, We investigated how starvation for nitrogen a ffects the general control regulatory system, because amino acid biosynthes is is part of nitrogen metabolism. Amino acid limitation results in the syn thesis of the central transcription factor Gcn4p, which binds to specific D NA-binding motif sequences called Gcn4-protein-responsive elements (GCREs) that are present in the promoter regions of its target genes. Nitrogen star vation increases GCN4 transcription but efficiently represses expression of both a synthetic GCRE6::lacZ reporter gene and the natural amino acid bios ynthetic gene ARO4. Repression of Gcn4p-regulated transcription by nitrogen starvation is independent of the ammonium sensing systems that include Mep 2p and Gpa2p or Ure2p and Gln3p but depends on the four upstream open readi ng frames in the GCN4 mRNA leader sequence. Efficient translation of GCN4 m RNA is completely blocked by nitrogen starvation, even when cells are simul taneously starved for amino acids and eukaryotic initiation factor-2 alpha is fully phosphorylated by Gcn2p. Our data suggest that nitrogen starvation regulates translation of GCN4 by a novel mechanism that involves the four upstream open reading frames but that still acts independently of eukaryoti c initiation factor-2 alpha phosphorylation by Gcn2p.