Secondary and tertiary structures of group I introns. (a) Group I introns share a common secondary structure in which short segments of the RNA strand fold back on themselves to form base-paired segments (exons shown in green; red arrows indicate splice sites). P segments are base-paired regions, numbered sequentially in the secondary structure; connections between P segments are shown by thinner lines indicating connectivity only, not actual sequence. The gray shaded box indicates parts of the structure that form the catalytic core, or active site, of the intron. (b) The crystal structure of the Tetrahymena group I intron P4-P6 domain (shown in (a) in yellow), the first large RNA structure to be solved, showing how the helices pack together to form a three-dimensional structure. (c) The complete group I intron, which forms an active site for substrate binding and transesterification.