Figure 4.

Structural motifs characterizing the closed, semi-open, and open conformations of the kt38 K-turn structure. Some key hydrogen-bond donor-acceptor distances are included as dotted lines. (A) Second A-minor interaction of kt38, from left to right: A-minor type I cis SE/SE A/C, A-minor type 0, and disruption of the second A-minor interaction. (B) Representative snapshots of the first A-minor interaction during kt38 opening. (C) Central loop (element K) between C- and NC-stems (A939 is shown as a thin line). From left to right: U1028 stacks on A939, U1028 alternatively stacks on A939 and U1027, and in the open conformation U1029 inserts into the central cavity, inducing stable stacking between A939, U1029, A1028, and G1027, respectively. The location of the structural motifs (shown in A–C) within the kt38 structure are indicated in D (encircled).