Figure 1.

K-turn sequences and classification. (A) The secondary structure of a simple, standard k-turn. Our standard nomenclature is used to designate nucleotide positions. The 3b•3n pair is frequently non-Watson–Crick. (B) A classification of k-turn structures. The k-turns can be divided into simple and complex. Examples are shown for each class. Simple k-turns are further divided into standard and non-standard, with the critical G•A pairs preserved or substituted, respectively. The complex k-turns exhibit a greater departure from the standard k-turn, where the primary sequence does not map onto the 3D RNA structure in a linear way. We now may consider the k-junctions discussed here as another branch of the complex k-turns.