Fig. 1. Secondary structure schemes and core sequences of hammerhead and pistol ribozymes. In each case the cleavage site is arrowed, and key nucleotides labelled.

A The core of the minimal hammerhead ribozyme structure. The helices and core nucleotides are labelled according to the standard nomenclature⁹. This has also been applied to the other hammerhead forms. B The extended hammerhead ribozyme^13,14. The loop-receptor interaction is indicated by the broken line. C The pseudoknot-containing hammerhead ribozyme¹⁹. We have termed the pseudoknot helix PK, and it’s stem helix IV. D The pistol ribozyme^19,20. The nomenclature for the helices is different for this ribozyme. There is no standard nomenclature for the nucleotide positions, so we have used those in the PDB file (ID 6R47) for this ribozyme.