The enzyme deoxyhypusine synthase catalyzes the initial step in the posttranslational production of the amino acid hypusine [N'-(4-amino-2-hydroxybutyl) ly~ inel. l,~ This step, the transfer of the 4-aminobutyl moiety of the polyamine spermidine to the€-amino group of a lysine residue to form the deoxyhypusine [Ne-(4-aminobuty1) lysinel residue, occurs at a single position in only one cellular protein, the eukaryotic translation initiation factor 5A ( ...