RNA 3-dimensional structural motifs as a critical constraint of viroid RNA evolution

What is the evidence for RNA 3D structural motifs constraining viroid evolution? Because some RNA 3D motifs control viral infection, strong selective pressures exist for maintaining the 3D motif structures that constrain the variation in sequence space. PSTVd loop 6 has a total of 46 possible sequence combinations, but there are only 8 functional variants out of 49 possible isosteric combinations [19]. [...]isostericity in RNA 3D motifs significantly reduced the sequence variations in PSTVd loop 6 by 84-fold (= 46/49) and testing for function by an additional factor of 6 (= 49/8), indicating that RNA 3D structural motifs serve as a critical constraining factor. Both substitutions are predicted to be isosteric with the original wild-type (WT) sequences [17]. [...]isosteric base substitutions in loop E can both maintain the local 3D structure and allow for the emergence of new infectious PSTVd variants. Despite differences in their genetic makeups and unique infection and evolution pathways, different viral and viroid RNAs should all share one common property: RNA 3D motif-based RNA-RNA, RNA-protein, and RNA-small ligand interactions necessary for completing life cycles [23-27]. [...]understanding how RNA 3D structural motifs play a role in viral infection and their exploration for regions of sequence space may potentially improve the prediction of outbreaks of new viruses.