The eIF5A protein is modified at a conserved lysyl residue by two enzymes to generate the unique amino acid hypusine, which is required for cell growth and viability in eukaryotes. See (Park, 2006) for a review of the hypusine biosynthesis pathway. PoxA catalyzes aminoacylation (with lysine) of the same conserved lysyl residue of EF-P (see Figure 4). YjeK, a 2,3-β-lysine aminomutase, is shown here modifying the Lys-Lys side-group to generate a fully active form of EF-P. Alternatively, YjeK may act on the lysine substrate prior to its ligation to EF-P.