Structure, folding and catalysis of the small nucleolytic ribozymes.

The small nucleolytic ribozymes are largely (but not exclusively) found in the RNA of plant pathogens and are involved in the self-catalysed processing of the concatameric RNA resulting from rolling circle replication. They catalyse a site-specific transesterification reaction in which their 2' hydroxyl attacks the 3' phosphate, with the exclusion of the 5' oxyanion. This requires an in-line geometry, which is not present in normal RNA structure. A significant part of the activation is probably provided by a distortion of the local conformation in order to facilitate the trajectory into the transition state and, thus, RNA folding and catalysis are intimately connected. A second element of the catalysis is provided by bound metal ions; however, a number of recent experiments cast doubt on the direct role of metal ions in the catalytic chemistry.