CRYSTAL-STRUCTURE OF A GROUP-I RIBOZYME DOMAIN - PRINCIPLES OF RNA PACKING

Group I self-splicing introns catalyze their own excision from precurs or RNAs by way of a two-step transesterification reaction. The catalyt ic core of these ribozymes is formed by two structural domains. The 2. 8-angstrom crystal structure of one of these, the P4-P6 domain of the Tetrahymena thermophila intron, is described. In the 160-nucleolide do main, a sharp bend allows stacked helices of the conserved core to pac k alongside helices of an adjacent region. Two specific long-range int eractions clamp the two halves of the domain together. a two-Mg2+-coor dinated adenosine-rich corkscrew plugs into the minor groove of a heli x, and a GAAA hairpin loop binds to a conserved 11-nucleotide internal loop. Metal- and ribose-mediated backbone contacts further stabilize the close side-by-side helical packing, The structure indicates the ex tent of RNA packing required for the function of large ribozymes, the spliceosome, and the ribosome.