THE RIBOSOMAL-RNA IDENTITY ELEMENTS FOR RICIN AND FOR ALPHA-SARCIN - MUTATIONS IN THE PUTATIVE CG PAIR THAT CLOSES A GAGA TETRALOOP

alpha-Sarcin is a ribonuclease that cleaves the phosphodiester band on the 3' side of G4325 in 285 rRNA; ricin A-chain is a RNA N-glycosidas e that depurinates the 5' adjacent A4324. These single covalent modifi cations inactivate the ribosome. An oligoribonucleotide that reproduce s 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 modificati on of the RNA by ricin, but not by alpha-sarcin, requires a Watson - C rick pair to shut off a putative GAGA tetraloop. Either the recognitio n elements for the two toxins are different despite their catalyzing c ovalent modification of adjacent nucleotides in 285 rRNA or there are transitions in the conformation of the alpha-sarcin/ricin domain in 28 S rRNA and one conformer is recognized by alpha-sarcin and the other b y ricin A-chain.