(Top, right) De-acetylation of proteins by sirtuins results in generation of O-acetyl-ADP-ribose which is cleaved to ADP-ribose and acetate by the hydrolases MacroD1/2, TARG1 and ARH3. (Top, left) De-phosphorylation of tRNA as intermediate step in tRNA splicing by Tpt1 family members releases mature tRNA and ADP-ribose-cyclic phosphate. The latter is cleaved to ADP-ribose and phosphate by the hydrolases Poa1P and MacroD1/2. (Bottom, left) The toxin MbcT of the type II TA system MbcTA functions as a NAD+ phosphorylase, thereby generating ADP-ribose-1″-phosphate. Binding of the antitoxin MbcA to MbcT inhibits toxin activity that is stimulated by inorganic phosphate. (Bottom, right) Mono-ADP-ribosylation of rifamycin by Arr enzymes leads to loss of the antibiotic activity of the molecule by inhibiting the binding of rifamycin to its target, i.e. the DNA-dependent RNA polymerase. (Ac: Acetate; ADPR: ADP-ribose; Ⓟ: phosphate; TA: Toxin-Antitoxin system).