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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.