title: Frequent occurrence of the T-loop RNA folding motif in ribosomal RNAs.
abstract: Analysis of atomic resolution structures of the rRNAs within the context of the 50S and the 30S ribosomal subunits have revealed the presence of nine examples of a recurrent structural motif, first observed in the TpsiC loop of tRNAs. The key component of this T-loop motif is a UA trans Watson-Crick/Hoogsteen base pair stacked on a Watson-Crick pair on one side. This motif is stabilized by several noncanonical hydrogen bonds, facilitating RNA-RNA as well as RNA-protein interactions. In particular, the sugar edge of the purine on the 3' side of the pivotal uridine in the UA pair frequently forms a noncanonical base pair with a distant residue. The bulged-out bases, usually seen as part of the motif, also use their Watson-Crick edges to interact with nearby residues via base-specific hydrogen bonds. In certain occurrences, a backbone reversal is stabilized by specific hydrogen bonds as is observed in the U-turn motifs and the adenosine residue of the key UA pair interacts with a third base via its Watson-Crick edge, essentially generating a base triple.
mesh_terms: D020029:Base Pairing; D001483:Base Sequence; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D012335:RNA, Ribosomal / Q000737:chemistry*; D012360:RNA, Transfer, Phe / Q000737:chemistry
chemical_list: D012335:RNA, Ribosomal; D012360:RNA, Transfer, Phe