The three-dimensional architecture of the class I ligase ribozyme

The class I ligase ribozyme catalyzes a Mg++-dependent RNA-ligation reaction that is chemically analogous to a single step of RNA polymerization. indeed, this ribozyme constitutes the catalytic domain of an accurate and general RNA polymerase ribozyme. The ligation reaction is also very rapid in both single- and multiple-turnover contexts and thus is informative for the study of RNA catalysis as well as RNA self-replication. Here we report the initial characterization of the three-dimensional architecture of the ligase. When the ligase folds, several segments become protected from hydroxyl-radical cleavage, indicating that the RNA adopts a compact tertiary structure. Ribozyme folding was largely, though not completely, Mg++ dependent, with a K-1/2[Mg] < 1 mM, and was observed over a broad temperature range (20degreesC-50degreesC). The hydroxyl-radical mapping, together with comparative sequence analyses and analogy to a region within 23S ribosomal RNA, were used to generate a three-dimensional model of the ribozyme. The predictive value of the model was tested and supported by a photo-cross-linking experiment.