The ribozyme core of group II introns is organized into six domains of secondary structure. Of these, domain II was long thought to be relatively unimportant for group II self-splicing. However, we now demonstrate the existence, in both major subdivisions of the group II family, of essential tertiary interactions involving domain II. theta-theta' is a novel tertiary interaction between the terminal loop of the IC1 stem of domain I and the basal stem of domain II. The theta-theta' interaction appears to stabilize the group II ribozyme core: it is essential for efficient self-splicing at elevated temperatures but, as shown by the use of a bimolecular reaction system, molecules with a defective theta-theta' contact are not affected in catalysis. An interaction, eta-eta', between domains II and VI of subgroup IIB introns was recently reported to mediate a conformational rearrangement between the two steps of the self-splicing reaction. We now show that domains II and VI of subgroup IIA introns also contact each other, although in a somewhat different way. Reinforcement of the eta-eta' interaction of a subgroup IIA intron prevents the use of a specific 2'-hydroxyl group in domain VI to initiate splicing by transesterification at the 5' splice site; the 5' intron-exon junction is hydrolyzed instead. Since disruption of eta-eta' has exactly opposite effects, and promotes reversal of the first transesterification step, it is concluded that formation of eta-eta' mediates a conformational change in subgroup IIA introns as well. Just like the eta-eta' interaction of subgroup IIB introns, the eta-eta' interaction of subgroup IIA introns (and the theta-theta' interaction) involves terminal loops of the GNRA family and their RNA receptors. Therefore, these motifs are used by nature not only to stabilize three-dimensional RNA architectures, but also in situations that require dynamic interactions.

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Journal Article | Research Support, Non-U.S. Gov't

Authors

Costa M, Déme E, Jacquier A, Michel F

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