The covalent self-assembly reaction by fragments of the Azoarcus group I intron ribozyme. (a) A schematic of the WXY (155 nt) + Z (45 nt) autocatalytic reaction, as first described in ref. [3]. The internal guide sequence (IGS) (red) of a WXYZ ribozyme (either in pieces or covalently contiguous) binds to the tag (orange) of a WXY fragment (blue) and catalyzes a recombination reaction with a Z fragment (green) to produce another WXYZ ribozyme, plus a free guanosine nucleotide. The reaction is autocatalytic; (b) A blowup of the IGS-tag interaction, where the middle nucleotides of the IGS (i.e., M) and of the tag (i.e., N) can be freely varied to all 16 possible combinations [5].