Reversibility of cyclization of the Tetrahymena rRNA intervening sequence: implication for the mechanism of splice site choice.

The Tetrahymena rRNA intervening sequence (IVS) excises itself from the pre-rRNA and then mediates its own cyclization. We now find that certain di- and trinucleotides with free 3' hydroxyl groups reopen the circular IVS at the cyclization junction, producing a linear molecule with the oligonucleotide covalently attached to its 5' end. This linear molecule recyclizes with release of the added oligonucleotide. Thus the IVS RNA, like an enzyme, lowers the activation energy for both forward and reverse cleavage-ligation reactions. Certain combinations of pyrimidines are required for circle reopening. The most reactive oligonucleotide is UCU. This sequence resembles those preceding the major and minor cyclization sites in the linear IVS RNA (UUU and CCU) and the 5' splice site in the pre-rRNA (UCU). We propose that an oligopyrimidine binding site within the IVS binds the sequences upstream of each of these target sites for cleavage-ligation.