Summary information and primary citation

PDB-id
7oa3
Class
RNA
Method
X-ray (2.8 Å)
Summary
Crystal structure of chili RNA aptamer in complex with dmhbo+ (iridium hexammine co-crystallized form)
Reference
Mieczkowski M, Steinmetzger C, Bessi I, Lenz AK, Schmiedel A, Holzapfel M, Lambert C, Pena V, Hobartner C (2021): "Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine." Nat Commun, 12, 3549. doi: 10.1038/s41467-021-23932-0.
Abstract
Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO<sub>+</sub> and DMHBI<sub>+</sub> ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.
G4 notes
2 G-tetrads, 1 G4 helix

Cartoon-block schematics in six views (download the tarball)

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List of 2 G-tetrads

 1 glyco-bond=---- sugar=-3-3 groove=---- planarity=0.328 type=other  N+ nts=4 GGGG 1:A.G9,1:A.G12,1:A.G42,1:A.G36
 2 glyco-bond=---- sugar=---3 groove=---- planarity=0.228 type=other  O+ nts=4 GGGG 1:A.G13,1:A.G32,1:A.G37,1:A.G41

List of 1 G4-helix

In DSSR, a G4-helix is defined by stacking interactions of G-tetrads, regardless of backbone connectivity, and may contain more than one G4-stem.

Helix#1, 2 G-tetrads, INTRA-molecular
 1  glyco-bond=---- sugar=-3-3 groove=---- WC-->Major N+ nts=4 GGGG 1:A.G9,1:A.G12,1:A.G42,1:A.G36
 2  glyco-bond=---- sugar=--3- groove=---- Major-->WC O+ nts=4 GGGG 1:A.G32,1:A.G13,1:A.G41,1:A.G37
  step#1  pp(><,inward)   area=5.67  rise=3.55 twist=-33.3
  strand#1 RNA glyco-bond=-- sugar=-- nts=2 GG 1:A.G9,1:A.G32
  strand#2 RNA glyco-bond=-- sugar=3- nts=2 GG 1:A.G12,1:A.G13
  strand#3 RNA glyco-bond=-- sugar=-3 nts=2 GG 1:A.G42,1:A.G41
  strand#4 RNA glyco-bond=-- sugar=3- nts=2 GG 1:A.G36,1:A.G37

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1 stacking diagram 
 1  glyco-bond=---- sugar=-3-3 groove=---- WC-->Major N+ nts=4 GGGG 1:A.G9,1:A.G12,1:A.G42,1:A.G36
 2  glyco-bond=---- sugar=--3- groove=---- Major-->WC O+ nts=4 GGGG 1:A.G32,1:A.G13,1:A.G41,1:A.G37
  step#1  pp(><,inward)   area=5.67  rise=3.55 twist=-33.3

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List of 4 non-stem G4-loops (including the two closing Gs)

 1 type=lateral   helix=#1 nts=4 GGAG 1:A.G9,1:A.G10,1:A.A11,1:A.G12
 2 type=lateral   helix=#1 nts=20 GGGCGCCAGUUCGCUGGUGG 1:A.G13,1:A.G14,1:A.G15,1:A.C16,1:A.G17,1:A.C18,1:A.C19,1:A.A20,1:A.G21,1:A.U22,1:A.U23,1:A.C24,1:A.G25,1:A.C26,1:A.U27,1:A.G28,1:A.G29,1:A.U30,1:A.G31,1:A.G32
 3 type=V-shaped  helix=#1 nts=6 GUUGGG 1:A.G32,1:A.U33,1:A.U34,1:A.G35,1:A.G36,1:A.G37
 4 type=lateral   helix=#1 nts=5 GUGCG 1:A.G37,1:A.U38,1:A.G39,1:A.C40,1:A.G41