title: Metal ion stabilization of the U-turn of the A37 N6-dimethylallyl-modified anticodon stem-loop of Escherichia coli tRNAPhe.
abstract: Nucleoside base modifications can alter the structures, dynamics, and metal ion binding properties of transfer RNA molecules and are important for accurate aminoacylation and for maintaining translational fidelity and efficiency. The unmodified anticodon stem-loop from Escherichia coli tRNA(Phe) forms a trinucleotide loop in solution, but Mg(2+) and dimethylallyl modification of A(37) N6 disrupt the loop conformation and increase the mobility of the loop and loop-proximal nucleotides. We have used NMR spectroscopy to investigate the binding and structural effects of multivalent cations on the unmodified and dimethylallyl-modified anticodon stem-loops from E. coli tRNA(Phe). The divalent cation binding sites were probed using Mn(2+) and Co(NH(3))(6)(3+). These ions bind along the major groove of the stem and associate with the anticodon loop on the major groove side in a nonspecific manner. Co(NH(3))(6)(3+) stabilizes the U-turn conformation of the loop in the dimethylallyl-modified molecule, and the chemical shift changes that accompany Co(NH(3))(6)(3+) binding are similar to those observed with the addition of Mg(2+). The base-phosphate and base-2'-OH hydrogen bonds that characterize the UNR U-turn motif lead to spectral signatures in the form of unusual (15)N and (1)H chemical shifts and reduced solvent exchange of the U(33) 2'-OH and N3H protons. The unmodified molecule also displays spectral features of the U-turn fold in the presence of Co(NH(3))(6)(3+), but the loop has additional conformations and is dynamic. The results indicate that charge neutralization by a polyvalent cation is sufficient to promote formation of the U-turn fold. However, base modification is necessary to destabilize competing alternative conformers even for a purine-rich loop sequence that is predicted to have strongly favorable base stacking energy.
authors.affiliation: Department of Biochemistry and Cell Biology, Rice University, Houston, TX 77251-1892, USA.
authors.forename: Javier
authors.initials: J
authors.lastname: Cabello-Villegas
authors.forename: Izabela
authors.initials: I
authors.lastname: Tworowska
authors.forename: Edward P
authors.initials: EP
authors.lastname: Nikonowicz
mesh_terms: D000225:Adenine / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism; D019883:Alkyl and Aryl Transferases / Q000737:chemistry; D000926:Anticodon / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002413:Cations, Divalent / Q000737:chemistry / Q000378:metabolism; D003035:Cobalt / Q000737:chemistry / Q000378:metabolism; D004926:Escherichia coli / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D007541:Isopentenyladenosine; D008274:Magnesium / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D019906:Nuclear Magnetic Resonance, Biomolecular; D009690:Nucleic Acid Conformation; D012329:RNA, Bacterial / Q000737:chemistry / Q000378:metabolism; D012360:RNA, Transfer, Phe / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics
chemical_list: D000926:Anticodon; D002413:Cations, Divalent; D012329:RNA, Bacterial; D012360:RNA, Transfer, Phe; C030862:cobalt ammonium complex; C001478:N(6)-(delta(2)-isopentenyl)adenine; D003035:Cobalt; D007541:Isopentenyladenosine; D019883:Alkyl and Aryl Transferases; C024274:adenylate isopentenyltransferase; D008274:Magnesium; D000225:Adenine