title: RNA internal loops with tandem AG pairs: the structure of the 5'GAGU/3'UGAG loop can be dramatically different from others, including 5'AAGU/3'UGAA.
abstract: Thermodynamic stabilities of 2 x 2 nucleotide tandem AG internal loops in RNA range from -1.3 to +3.4 kcal/mol at 37 degrees C and are not predicted well with a hydrogen-bonding model. To provide structural information to facilitate development of more sophisticated models for the sequence dependence of stability, we report the NMR solution structures of five RNA duplexes: (rGACGAGCGUCA)(2), (rGACUAGAGUCA)(2), (rGACAAGUGUCA)(2), (rGGUAGGCCA)(2), and (rGACGAGUGUCA)(2). The structures of these duplexes are compared to that of the previously solved (rGGCAGGCC)(2) (Wu, M., SantaLucia, J., Jr., and Turner, D. H. (1997) Biochemistry 36, 4449-4460). For loops bounded by Watson-Crick pairs, the AG and Watson-Crick pairs are all head-to-head imino-paired (cis Watson-Crick/Watson-Crick). The structures suggest that the sequence-dependent stability may reflect non-hydrogen-bonding interactions. Of the two loops bounded by G-U pairs, only the 5'UAGG/3'GGAU loop adopts canonical UG wobble pairing (cis Watson-Crick/Watson-Crick), with AG pairs that are only weakly imino-paired. Strikingly, the 5'GAGU/3'UGAG loop has two distinct duplex conformations, the major of which has both guanosine residues (G4 and G6 in (rGACGAGUGUCA)(2)) in a syn glycosidic bond conformation and forming a sheared GG pair (G4-G6*, GG trans Watson-Crick/Hoogsteen), both uracils (U7 and U7*) flipped out of the helix, and an AA pair (A5-A5*) in a dynamic or stacked conformation. These structures provide benchmarks for computational investigations into interactions responsible for the unexpected differences in loop free energies and structure.
authors.affiliation: Department of Chemistry, University of Rochester, Rochester, New York 14627-0216, USA.
authors.forename: Nicholas B
authors.initials: NB
authors.lastname: Hammond
authors.forename: Blanton S
authors.initials: BS
authors.lastname: Tolbert
authors.forename: Ryszard
authors.initials: R
authors.lastname: Kierzek
authors.forename: Douglas H
authors.initials: DH
authors.lastname: Turner
authors.forename: Scott D
authors.initials: SD
authors.lastname: Kennedy
mesh_terms: D001483:Base Sequence; D006151:Guanosine / Q000737:chemistry*; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D009711:Nucleotides; D011810:Quinoxalines; D012313:RNA / Q000737:chemistry*; D012996:Solutions; D013816:Thermodynamics; D014498:Uracil
chemical_list: D009711:Nucleotides; D011810:Quinoxalines; D012996:Solutions; D006151:Guanosine; D014498:Uracil; D012313:RNA; C017738:TANDEM (quinoxaline)