Novel features of the functional site and expression of the yeast deoxyhypusine synthase

A unique amino acid, hypusine, is formed posttranslationally in the precursor of eukaryotic translation initiation factor 5A (eIF-5A). Deoxyhypusine synthase catalyzes the first of two steps in the biosynthesis of hypusine. We reported earlier that the DYS1 gene encoding deoxyhypusine synthase is essential for cell viability and proliferation in yeast. Here, we show by deletion studies that both N- and C-terminal regions, which are not so well conserved, are necessary for the activity of the yeast enzyme. Of the seven cysteine residues present in the yeast enzyme, only one cysteine (position 252; C252) appeared to be essential for its activity. Moderate overexpression of DYS1 showed very little effects on cell growth and no obvious effects on the intracellular level of eIF-5A. However, repression of the expression of DYS1 resulted in near-complete depletion of eIF-5A 24 h after the initiation of repression and was followed by cell growth arrest after another 24 h. This novel finding suggests that the major role of deoxyhypusine synthase in cell proliferation is mediated not only through its modification of the eIF-5A precursor, but also through its regulation of intracellular eIF-5A levels.