Studies on ribosomal RNA topography play an important role toward elucidation of the functional roles of rRNAs in protein synthesis.RNA N-glycosidases are a group of ribosome-inactivating proteins which inactivate ribosomes by hydrolyzing the N-C glycosidic bond of a special adenylic acid in rRNA and releasing the adenine. Ricin A-chain is the first determined RNA Nglycosidase which has been studied in the greatest detail. UP to the present. 25 glycosidases have been characterized. The action site of RNA N-glycosidases lies in the a-sarcin domain of 28S rRNA. Treatment of RNA N-glycosidases induces conformational change of ribosomes and leads to the inactivation.