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-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 alpha-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 285 rRNA or there are transitions in the conformation of the alpha-sarcin/ricin domain in 28S rRNA and one conformer is recognized by alpha-sarcin and the other by ricin A-chain.