An uncommon [K⁺(Mg²⁺)₂] metal ion triad imparts stability and selectivity to the Guanidine-I riboswitch

Thermal melting analysis of Guanidine-I riboswitch. Thermal denaturation of the Burkholderia Guanidine-I riboswitch was performed in the presence of 0.5 mM MgCl₂ and either 50 mM NaCl (gray) or 50 mM KCl (yellow) and (A) no ligand, (B) 250 µM urea, (C) 50 µM guanidine, (D) 150 µM guanidine, or (E) 250 µM guanidine. Error bars represent standard error of three independent measurements. Melting temperatures (T_m) for the tertiary structure unfolding transition (K⁺ data only) are marked with a blue line. Insets show zoomed-in view of 35°C–55°C regions.