Diphtheria toxin and some other bacterial toxins catalyse this reaction, which inactivates translation elongation factor 2 (EF2). The acceptor is diphthamide, a unique modification of a histidine residue in the elongation factor found in archaebacteria and all eukaryotes, but not in eubacteria. cf. EC 18.104.22.168 NAD(P)+—protein-arginine ADP-ribosyltransferase. The relevant histidine of EF2 is His715 in mammals, His699 in yeast and His600 in Pyrococcus horikoshii.