Phylogenetic and genetic evidence for base-triples in the catalytic domain of group I introns

Abstract

UNDERSTANDING the mechanisms by which ribozymes catalyse chemical reactions requires a detailed knowledge of their structure. The secondary structure of the group I introns has been confirmed by comparison of over 70 published sequences^1–4, by chemical protection studies⁵, and by genetic experiments involving compensatory mutations^2,6,7. Phylogenetic data can also be used to identify tertiary interactions in RNA molecules. This was first done by Levitt⁸, who predicted tertiary interactions in transfer RNA, which were subsequently confirmed by X-ray crystallography⁹. More recently, sequence comparison data have been used to predict tertiary interactions in ribosomal RNA¹⁰. We have searched a complete alignment of the core regions of group I introns^1,2 for evolutionary covariations that could not be ascribed to classical Watson–Crick or wobble base pairings. Here we describe two examples of phylogenetic covariation that are most simply explained by postulating hydrogen-bonded base-triples similar to those found in tRNA. Genetic experiments with the Tetrahymena and sunY introns confirm the importance of these interactions for the structure of the ribozyme.