GCN1, A TRANSLATIONAL ACTIVATOR OF GCN4 IN SACCHAROMYCES-CEREVISIAE, IS REQUIRED FOR PHOSPHORYLATION OF EUKARYOTIC TRANSLATION INITIATION FACTOR-II BY PROTEIN-KINASE GCN2

Phosphorylation of the alpha subunit of eukaryotic translation initiat ion factor 2 (eIF-2alpha) by the protein kinase GCN2 mediates increase d translation of the transcriptional activator GCN4 in amino acid-star ved yeast cells. We show that this key phosphorylation event and the a ttendant translational induction of GCN4 are dependent on the product of a previously uncharacterized gene, GCN1. Inactivation of GCN1 did n ot affect the level of eIF-2alpha phosphorylation when mammalian eIF-2 alpha kinases were expressed in yeast cells in place of GCN2, arguing against an involvement of GCN1 in dephosphorylation of eIF-2alpha. In addition, while GCN1 is required in vivo for phosphorylation of eIF-2a lpha by GCN2, cell extracts from gcn1DELTA strains contained wild-type levels of GCN2 eIF-2alpha-kinase activity. On the basis of these resu lts, we propose that GCN1 is not needed for GCN2 kinase activity per s e but is required for in vivo activation of GCN2 in response to the st arvation signal, uncharged tRNA. GCN1 encodes a protein of 297 kDa wit h an 88-kDa region that is highly similar in sequence to translation e longation factor 3 identified in several fungal species. This sequence similarity raises the possibility that GCN1 interacts with ribosomes or tRNA molecules and functions in conjunction with GCN2 in monitoring uncharged tRNA levels during the process of translation elongation.