The complete VS ribozyme comprises seven helical segments, connected by three three-way RNA junctions. In the presence of Mg 2+ ions, cleavage occurs within the internal loop of helix I. This requires the participation of a guanine (G638) within the helix I loop, and a remote adenine (A756) within an internal loop of helix VI. Previous structural studies have suggested that helix I docks into the fold of the remaining part of the ribozyme, bringing A756 and G638 close to the scissile phosphate to allow the cleavage reaction to proceed. We show here that while either A756C or G638A individually exhibit very low cleavage activity, a mixture of the two variants leads to cleavage of the A756C RNA, but not the G638A RNA. The rate of cleavage depends on the concentration of the VS G638A RNA, as expected for a bimolecular interaction. This regaining of cleavage activity by complementation indicates that helix I of one VS RNA can interact with another VS RNA molecule to generate a functional active site in trans.

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