The toxin then transfers an ADP-ribose from NAD+ to a diphthamide residue, a modified histidine (amino acid), which is found within the EF-2 protein. EF-2 is needed for translocation of tRNA from the A-site to the P-site of the ribosome during traslation. The ADP-ribosylation is reversible by administering high concentrations of nicotinamide, one of the reaction products.
Nicotinamide aka nicotinic acid amide, is the amide of nicotinic acid (vitamin B3).