The guanosine binding mechanism of the Tetrahymena group I intron

The <it>Tetrahymena</it> group I intron catalyzes self-splicing through two consecutive transesterification reactions, using a single guanosine-binding site (GBS). In this study, we constructed a model RNA that contains the GBS and a conserved guanosine nucleotide at the 3′-terminus of the intron (ωG). We determined by NMR the solution structure of this model RNA, and revealed the guanosine binding mechanism of the group I intron. The G22 residue, corresponding to ωG, participates in a base triple, G22··G3·C12, hydrogen-bonding to the major groove edge of the Watson-Crick G3·C12 pair. The G22 residue also interacts with A2, which is semi-conserved in all sequenced group I introns.