Alepuz, P.; Prieto-Diez, S.; Aguilar-Diez, A.; Araque, L.; Peris, D.

eIF5A is an evolutionarily conserved protein found in all eukaryotes, and in Archaea and bacteria. It functions as a translation factor promoting ribosomal elongation, and it can also bind to genes in the nucleus and to mRNA. eIF5A is encoded by two paralogous genes in most eukaryotes, with one copy (TIF51A in yeast and EIF5A1 in humans) highly expressed and essential, and the other (TIF51B in yeast and EIF5A2 in humans) repressed in most cells and conditions. eIF5A is linked to viral infection, aging, diabetes and neurodevelopmental disorders. Whilst derepression of the duplicated silent gene EIF5A2 is associated with several cancers, promoting metastasis. To expand our knowledge of eIF5As function, we searched for suppressors of yeast temperature-sensitive mutants of TIF51A, which cannot grow at restrictive temperature. All suppressors contained mutations in the transcriptional repressors Rox1 and Mot3, resulting in the upregulation of the paralogous gene TIF51B. Next, we searched for suppressors of TIF51A temperature-sensitive mutants in yeast lacking the TIF51B gene. The frequency of suppression was 20-times lower and all suppressors were revertants or contained intragenic mutations in the Tif51A protein that conferred stability. Our results suggest that the duplicated eIF5A gene serves as a non-conventional backup system that rescues mutations in the first copy, but acting at the population level. Furthermore, our results expand our understanding of the repression mechanisms that keep the second eIF5A gene silent. Lastly, our results demonstrate that eIF5A is an indispensable protein in eukaryotic cells, whose function cannot be substituted by mutations in other proteins or pathways.