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α-Sarcin is a ribonuclease that cleaves the phosphodiester bond on the 3′ side of G4325 in 28S rRNA; ricin A-chain is a RNA N-glycosidase that depurinates the 5′ adjacent A4324. These single covalent modifications Inactivate the ribosome. An oligoribonucleotide that reproduces the structure of the sarcin/ricin domain in 28S rRNA was synthesized and mutations were constructed in the 5′ C and the 3′ G that surround a GAGA tetrad that has the sites of toxin action. Covalent modification of the RNA by ricin, but not by α-sarcin, requires a Watson-Crick pair to shut off a putative GAGA tetraloop. Either the recognition elements for the two toxins are different despite their catalyzing covalent modification of adjacent nucleotides in 28S rRNA or there are transitions in the conformation of the α-sarcin/ricin domain in 28S rRNA and one conformer is recognized by α-sarcin and the other by ricin A-chain.