Guanosine binding required for cyclization of the self-splicing intervening sequence ribonucleic acid from Tetrahymena thermophila.

We have converted the intramolecular cyclization reaction of the self-splicing intervening sequence (IVS) ribonucleic acid (RNA) from Tetrahymena thermophila into an intermolecular guanosine addition reaction. This was accomplished by selectively removing the 3'-terminal nucleotide by oxidation and beta-elimination; the beta-eliminated IVS thereby is no longer capable of reacting with itself. However, under cyclization conditions, a free guanosine molecule can make a nucleophilic attack at the normal cyclization site. We have used this guanosine addition reaction as a model system for a Michaelis-Menten kinetic analysis of the guanosine binding site involved in cyclization. The results indicate that functional groups on the guanine that are used in a G-C Watson-Crick base pair are important for the cyclization reaction. This is the same result that was obtained for the guanosine binding site involved in splicing [Bass, B. L., & Cech, T. R. (1984) Nature (London) 308, 820-826]. Unlike splicing, however, certain additional nucleotides 5' to the guanosine moiety make significant binding contributions. We conclude that the guanosine binding site in cyclization is similar to, but not identical with, the guanosine binding site in splicing. The same binding interactions used in cyclization could help align the 3' splice site of the rRNA precursor for exon ligation. We also report that the phosphodiester bond at the cyclization site is susceptible to a pH-dependent hydrolysis reaction; the phosphodiester bond is somehow activated toward attack by the 3'hydroxyl of a guanosine molecule or by a hydroxyl ion.