The structure and active site of the Varkud satellite ribozyme.

The Varkud satellite ribozyme is the largest of the small nucleolytic ribozymes, and the only one for which there is no crystal structure. It can be divided into a trans -acting ribozyme, consisting of five helices organized by two three-way helical junctions, and a stem-loop substrate with which it interacts, primarily by tertiary interactions. We have determined the global fold of the ribozyme, and the manner by which it interacts with the substrate. The substrate interacts with a cleft formed between helices II and VI (organized by the lower helical junction), where it contacts the A730 loop, the probable active site of the ribozyme. Within this loop, there is a critical adenine base (A756) that is a candidate for direct nucleobase participation in the cleavage reaction.