A tertiary interaction that links active-site domains to the 5 ' splice site of a group II intron

Group II introns are self-splicing RNAs that are commonly found in the gene s of plants, fungi, yeast and bacteria(1,2). Little is known about the tert iary structure of group II introns, which are among the largest natural rib ozymes. The most conserved region of the intron is domain 5 (D5), which, to gether with domain 1 (D1), is required for all reactions catalysed by the i ntron(3). Despite the importance of D5, its spatial relationship and tertia ry contacts to other active-site constituents have remained obscure. Furthe rmore, D5 has never been placed directly at a site of catalysis by the intr on. Here we show that a set of tertiary interactions (lambda-lambda') links catalytically essential regions of D5 and D1, creating the framework for a n active-site and anchoring it at the 5' splice site. Highly conserved elem ents similar to components of the lambda-lambda' interaction are found in t he eukaryotic spliceosome.