THE CRYSTAL-STRUCTURE OF AN ALL-RNA HAMMERHEAD RIBOZYME - A PROPOSED MECHANISM FOR RNA CATALYTIC CLEAVAGE

We have solved the crystal structure of an all-RNA hammerhead ribozyme having a single 2'-O-methyl cytosine incorporated at the active site to prevent cleavage. The conditions used differ from those in another recent solution in four significant ways: first, it is an all-RNA ribo zyme rather than a DNA-RNA hybrid; second, the connectivity of the rib ozyme backbone strands is different; third, the crystals were grown in the presence of a much lower concentration of salt; and fourth, the c rystal packing scheme is very different. Nevertheless, the three-dimen sional structure of the all-RNA hammerhead ribozyme is similar to the previous structure. Five potential Mg(II)-binding sites are Identified , including one positioned near the ribozyme catalytic pocket. Upon th is basis, as well as upon comparisons with the metal-binding sites in the structurally homologous uridine turn of tRNA(Phe), We propose a me chanism for RNA catalytic cleavage.