A small metalloribozyme with a two-step mechanism.

An RNA molecule consisting of an asymmetric internal loop of six nucleotides can be rapidly and specifically cleaved by Pb2+ in the presence of Mg2+. The 5' cleavage product terminates with a 3' phosphomonoester generated from a 2',3'-cyclic phosphodiester reaction intermediate. This two-step reaction mechanism resembles that of many protein ribonucleases but has not previously been observed for reactions catalysed by RNA.