Two guanosine binding sites exist in group I self-splicing IVS RNAs

A shortened form of the self-splicing rRNA intervening sequence (IVS) of Tetrahymena thermophila can catalyze a transesterification reaction, termed G-exchange, between a monomeric guanosine derivative such as GTP and the substrate GpN (where N is A, C, G or U). The reaction is specific to the two guanosines involved, providing evidence that two guanosine binding sites exist in this group I IVS RNA. One binding site accommodates a guanosine which initiates self-splicing and the other recognizes the guanosine preceding the 3' splice site. Previously, only one guanosine binding site was thought to be involved in the mechanism of self-splicing. Based on the two functionally distinguishable guanosine binding sites, a new model is proposed to explain how the two independent transesterification reactions required for self-splicing might proceed in a concerted manner.