Fig. 3.

A classification of K-turn structures. 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. k-junctions can be considered as another branch of the complex K-turns. This scheme is taken from reference [13].