EFFECT OF INITIATION-FACTOR EIF-5A DEPLETION ON PROTEIN-SYNTHESIS ANDPROLIFERATION OF SACCHAROMYCES-CEREVISIAE

Eukaryotic translation initiation factor eIF-5A (formerly eIF-4D) is t hought to function in protein synthesis by promoting synthesis of the first peptide bond because it stimulates methionyl-puromycin formation in vitro. eIF-5A is encoded by two genes (TIF51A and TIF51B) in Sacch aromyces cerevisiae; the protein and its hypusine modification are ess ential for cell viability. To analyze the factor's function in vivo, w e expressed from the repressible GAL promoter a functional but unstabl e eIF-5A fusion protein (R-eIF-5A) with an NH2-terminal arginine which is subject to rapid turnover through the NH2-terminal end rule proteo lytic pathway. When the conditional mutant strain is shifted from gala ctose to glucose medium, the rapid disappearance of R-eIF-5A protein o ccurs within one generation, causing an immediate inhibition of cell g rowth. However, eIF-5A-depleted cells synthesize protein at about 70% of the wild type rate and exhibit only a slight change in polysome pro files reflecting a subtle defect in a late step of translation initiat ion. These results suggest that the activity of eIF-5A may not be abso lutely essential for general protein synthesis. Rather, eIF-5A may be selectively required for translation of certain mRNAs and/or may be in volved in some other aspect of cell metabolism.