1. Department of Biochemistry and Molecular Biophysics and
2. Howard Hughes Medical Institute, Columbia University, New York, New York 10032 , USA
3. Present address: Centre de Biophysique Moleculaire, CNRS, Rue Charles Sadron, 45071 Orleans, Cedex 2, France.

Correspondence to: Anna Marie Pyle^1,3 Correspondence and requests for material should be addressed to A.M.P. (e-mail: Email: amp11@columbia.edu).

Abstract

Group II introns are self-splicing RNAs that are commonly found in the genes of plants, fungi, yeast and bacteria^{1, 2}. Little is known about the tertiary structure of group II introns, which are among the largest natural ribozymes. The most conserved region of the intron is domain 5 (D5), which, together with domain 1 (D1), is required for all reactions catalysed by the intron³. Despite the importance of D5, its spatial relationship and tertiary contacts to other active-site constituents have remained obscure. Furthermore, D5 has never been placed directly at a site of catalysis by the intron. Here we show that a set of tertiary interactions (–') links catalytically essential regions of D5 and D1, creating the framework for an active-site and anchoring it at the 5' splice site. Highly conserved elements similar to components of the –' interaction are found in the eukaryotic spliceosome.