Fig. 7: G12(S) forms G-tetrads and may have a left-handed G-quadruplex structure similar to Block2Δ.

a The four guanine form a planar G-tetrad structure through Hoogsteen hydrogen bonds and are stabilized by alkali metal ions (e.g., K⁺ and Na⁺). Guanosine-rich RNA oligomers form several G-tetrads with a parallel orientation to form G-quadruplex structures. The structure of Block2Δ¹⁵ (GTGGTGGTGGTG) consists of two subunits of two-layered parallel-strand G-quadruplex structure with left-handed helicity. b CD spectra of G12(S) in 10 mM Tris/HCl (pH 7.0) (black curve), 30 mM NaCl (blue curve) or 30 mM KCl (yellow curve). G12(S) (4 μM) was incubated overnight in 10 mM Tris/HCl, and then further incubated overnight with 30 mM NaCl or 30 mM KCl. c Nucleotide sequences of G12(S) and BL2(S). The Tm of G12(S) in 30 mM KCl or 30 mM NaCl in 10 mM Tris/HCl (pH7.0) obtained for the first and second temperature increases are shown (see Supplementary Fig. 12). Helicity of G-quadruplexes determined by CD spectroscopy are shown as left-handed parallel (L) and right-handed parallel (R). d CD spectra of BL2(S). BL2(S) was incubated in H₂O overnight (blue curve), and then further incubated in Na-K phosphate buffer (orange curve), 30 mM KCl-10 mM Tris/HCl (pH 7.0; yellow curve) or 30 mM NaCl-10 mM Tris/HCl (pH 7.0) (gray curve) for 1 h. e, f Mobility shift of G12(S) and BL2(S) in native PAGE. G12(S) and BL2(S) at 4 μM (20 pmols) pretreated with 0, 5, 10, 20, or 40 μM of NMM and then with 10 mM KCl-Tris/HCl for 20 min were separated on a 15% PAGE at a constant voltage of 120 mV for 100 min using Tris/borate containing 10 mM KCl (e) or 10 mM NaCl (f). The gels were stained with NMM. The dsRNA marker in the rightmost lane of each gel was pretreated with 200 pmol of NMM prior to PAGE, whereas the second lane from the left was not, which was detected by subsequent DAPI staining (see Supplementary Fig. 11c). g Effects of MNN on CD spectra of G12(S), BL2(S) and ADAM10(S). These PS-RNA were incubated overnight in H₂O, and then incubated with 0, 10 and 20 μM NMM in 10 mM Tris/HCl (pH 7.0) for 30 min and further incubated with 30 mM KCl for 1 h. CD spectra of 0 μM NMM (blue curve), 10 μM NMM (orange curve) and 20 μM NMM (gray curve) are shown. h Possible structural changes induced by NMM and NaCl in the G-quadruplex dimer structure of BL2(S), which may be a structural model for G12(S). NMM and/or NaCl convert the left-handed parallel dimer of BL2(S) formed in the presence of K⁺ into a right-handed parallel monomer.