a, The continuous stacking of stem P1, pseudoknot stem T1, stem P2, pseudoknot stem T2 and stem P3 (on proceeding from bottom to top). b–e, Stacking of adjacent base pairs at the junction of helical stems. Stacking between Watson-Crick G25-C14 (P3) and G36-C13 (T2) pairs (panel b), between Watson-Crick G12-C37 (T2) and A11-U44 (P2) pairs (panel c), between non-canonical sheared cis Hoogsteen-sugar edge A7•G48 and trans Watson-Crick A34•A49 pairs (panel d), and between Watson-Crick G52-C31 pair and G53 (panel e).