FIGURE 1.

The VS ribozyme sequence, and a possible cleavage reaction in trans. (A) The sequence of the VS ribozyme, with the critical nucleotides for catalytic activity A756 and G638 highlighted. Ribozyme cleavage occurs within the loop of helix I, at the position arrowed. The sequence shown includes the additional helix VII. (B) A model of the folded VS ribozyme based on small-angle X-ray scattering (Lipfert et al. 2008). The helices are colored in the same manner as for the sequence in A. (C) The principle of a trans reaction between ribozymes. The normal reaction occurs in cis, resulting from an interaction between the loops of helices I and VI (left). However, it is formally possible that helix I of one ribozyme molecule could interact with helix VI of a different molecule to generate a functional active site in trans (right). No attempt has been made to represent the three-dimensional structure of the ribozyme in these schemes.