Enhanced NMR signal detection of imino protons in RNA molecules containing 3' dangling nucleotides.

We present a method for improving the quality of nuclear magnetic resonance (NMR) spectra involving exchangeable protons near the base of the stem of RNA hairpin molecules. NMR spectra of five different RNA hairpins were compared. These hairpins consisted of a native RNA structure and four molecules each having different unpaired, or dangling, nucleotides at the 3' end. NMR experiments were acquired in water for each construct and the quality of the imino proton spectral regions were examined. The imino resonances near the base of the stem of the wild type RNA structure were not observed due to breathing motions. However, a significant increase in spectral quality for molecules with dangling 3' adenosine or guanosine nucleotides was observed, with imino protons detected in these constructs that were not observed in the wild type construct. A modest improvement in spectral quality was seen for the construct with a 3' unpaired uridine, whereas no significant improvement was observed for a 3' unpaired cytidine. This improvement in NMR spectral quality mirrors the increased thermodynamic stability observed for 3' unpaired nucleotides which is dependant on the stacking interactions of these nucleotides against the base of the stem. The use of a dangling 3' adenosine nucleotide represents an easy method to significantly improve the quality of NMR spectra of RNA molecules.

The three-dimensional structure of the Moorella thermoacetica selenocysteine insertion sequence RNA hairpin and its interaction with the elongation factor SelB.

Incorporation of the amino acid selenocysteine into a growing protein chain involves the interaction between a hairpin in the mRNA termed the selenocysteine insertion sequence (SECIS) and the special elongation factor SelB. Here we present the structure of the SECIS from the thermophilic organism Moorella thermoacetica (SECIS-MT) determined using nuclear magnetic resonance (NMR) spectroscopy. The SECIS-MT hairpin structure contains a pentaloop with the first and fourth nucleotides of the loop forming a noncanonical GC base pair; the fifth loop nucleotide is bulged out and unstructured. The G and U in positions two and three are on opposite sides of the loop and solvent exposed. The backbone resonances of the SECIS-binding domain from the M. thermoacetica SelB protein were assigned, and the degree of chemical shift perturbations that occur upon SECIS binding were mapped onto the structure of the complex. We demonstrate that a region in the third winged-helix domain of SelB, not previously implicated in binding, is affected by SECIS binding.

The NMR and X-ray structures of the Saccharomyces cerevisiae Vts1 SAM domain define a surface for the recognition of RNA hairpins.

The SAM domain of the Saccharomyces cerevisiae post-transcriptional regulator Vts1 has a high affinity towards RNA hairpins containing a CUGGC pentaloop. We present the 1.6 Angstroms X-ray crystal structure of the Vts1 SAM domain in its unliganded state, and the NMR solution structure of this domain in its RNA-bound state. Both structures reveal a canonical five helix SAM domain flanked by additional secondary structural elements at the N and C termini. The two structures are essentially identical, implying that no major structural rearrangements occur upon RNA binding. Amide chemical shift changes map the RNA-binding site to a shallow, basic patch at the junction of helix alpha5 and the loop connecting helices alpha1 and alpha2.