P2.1 in LC ribozymes. (A) Secondary structure diagrams of P2.1 in Allovahlkampfia (AP2.1), Naegleria (NP2.1) and Didymium (DP2.1) LC ribozymes. Yellow boxes: identical nucleotide positions in P2.1 and P10 of all three LC ribozyme variants. Red boxes: compensatory base-pair changes in P2.1 observed between the AspLC and NprLC ribozymes. Note that pHEG-1 in AspLC and NprLC has a similar location to that of the L9-tetraloop DP2 receptor interaction in DirLC , and that DP2.1 is a regular hairpin structure, while AP2.1/NP2.1 is made by a LC:HE mRNA interaction. (B) Structure model of the pre-cleaved AspLC ribozyme. Coloured sequence segments in the secondary structure (left) correspond to those in the 3D structure model (right). The HEG sequence (red line, dot) is involved in AP2.1 (yellow box).