ABSTRACT
The solution of structures of ribosomal subunits greatly increased the database of RNA structures at a stroke, and one of the first motifs to be identified as a repeated structural element was the kink turn, generally abbreviated to k-turn. Steitz and colleagues found six examples in the 23S rRNA of Haloarcula marismortui, and two further cases in the 16S rRNA of Thermus thermophilus (Klein et al., 2001). K-turns were also independently noted as a repeated sequence in regulatory elements in RNA (Winkler et al., 2001). The k-turn occurs in duplex regions of RNA, generally comprising a three-nucleotide bulge followed on the 3′ side by a G•A pair, an A•G pair and frequently one or two further non-WatsonCrick pairs before reverting to regular basepairing (Figure 3.1a). This helix is termed the NC (in reference to the noncanonical basepairing), whereas the conventionally basepaired helix on the 5′ side of the bulge is called the C (i.e., canonical) helix. The name of the motif derives from the ~50° included angle subtended between the axes of the C and NC helices (Figure 3.1b).
