Final answer:
In yeast cells during amino acid starvation, GCN2 kinase activity increases, leading to the phosphorylation of eIF2 and eIF2B inactivation. This inhibits general protein synthesis while specifically increasing the translation of GCN4, which up-regulates amino acid biosynthesis pathways to aid cell survival.
Step-by-step explanation:
During amino acid starvation, specific regions and mechanisms within yeast cells are activated to cope with the stress of nutrient scarcity. When GTP-eIF2 cannot be regenerated efficiently, and TCs (ternary complexes) are in short supply, a chain reaction is initiated. The central player in this process is GCN2 kinase which, upon activation, phosphorylates eIF2, leading to the phosphorylation and subsequent inactivation of eIF2B, thus preventing the exchange of GTP for GDP on eIF2. This inactivation inhibits general protein synthesis but paradoxically up-regulates the translation of a transcription factor named GCN4.
GCN4 is crucial because it drives the up-regulation of amino acid biosynthesis pathways, essentially directing the cell to synthesize its own amino acids. The translation of GCN4 is enhanced through a sophisticated mechanism involving upstream open reading frames (uORFs) in the 5' untranslated region (UTR) of its mRNA which allows GCN4 translation under amino acid-deprived conditions. This process ensures that, even with inhibited protein synthesis overall, the synthesis of enzymes required for amino acid production is maintained, helping the cell survive during periods of nutrient scarcity.