FIGURE 1. Sequence and secondary structure of ligase ribozymes containing either three or two different nucleotide subunits.

a, Ribozyme containing D, G and U residues, which was made to react with a substrate containing only A and U. This structure is supported by chemical modification and site-directed mutagenesis studies⁹. Bold G at positions 1, 58 and 63 indicates residues that could not be replaced by D or U without complete loss of catalytic activity. b, Ribozyme containing only D and U, which was made to react with a substrate containing only D and U. This structure is conjectural. Note that this molecule is shortened by one nucleotide at the 5' end and lengthened by six nucleotides at the 3' end compared with the ribozyme shown in a.