The λ-Turn: A New Structural Motif in Ribosomal RNA

RNA structural motifs are recurrent structural elements occurring in RNA molecules. They play essential roles in consolidating RNA tertiary structures and in binding proteins. Recently, we identified a new type of RNA structural motif, namely λ-turn, from ribosomal RNAs. This motif has a helix-internal loop-helix structure. The directions of its two helices are changed *90° due to the existence of the internal loop. A guanine from the 3′-end of the internal loop extrudes out and forms a base triple with a G-C WC base pair from one helix of the motif. From the global perspective, the λ-turn is often capped by a helix and a tetraloop. A nucleotide between the capped helix and the tetraloop forms a consecutive base triple next to the first one with a G-C pair from the same helix that the first G-C pair resides. The λ-turn motif has a consensus sequence pattern and its 3D structure is conserved across different species. All the identified λ-turns are located on surfaces of ribosomal RNAs. Structures of ribosomes reveal direct interactions between λ-turns and ribosomal proteins. All these observations indicate that λ-turns have an important role in binding with ribosomal proteins.