Fig. 1.

The sequence of standard K-turns. The sequence of H. marismortui Kt-7 is shown at the top, with the standard nomenclature for nucleotide positions [24]. The C and NC helices are indicated, the latter having the G·A pairs. The loop nucleotides are designated Ln with n increasing 5′ to 3′. The remaining nucleotide positions are indicated by the suffix b for the bulge-containing strand, and n for the other strand, and positions 3′ to the bulge (i.e. the NC helix) are positive and 5′ to the bulge (C helix) are negative. The sequences of the SAM-I, cyclic-diGMP and cobalamine riboswitch K-turns are shown. Each is a standard, simple K-turn.