Small catalytic RNAs. (A) Hammerhead ribozyme split into substrate and catalyst,51 basepaired by means of two flanking arms (in bold). Numbering is that of Hertel et al.52 (B) Hairpin ribozyme split into substrate and catalyst.42 (C) Pseudoknot motif of the HDV ribozyme.45 Arrows show position of scissile bonds. IUB codes used throughout: N = A, C, G, U; R = A, G; Y = C, U; B = C, G, U; D = A, U, G; H = A, C, U; V = A, C, G.