Ni2+-binding RNA motifs with an asymmetric purine-rich internal loop and a G-A base pair

RNA molecules with high affinity for immobilized Ni2+ were isolated from an RNA pool with 50 randomized positions by in vitro selection-amplification. The selected RNAs preferentially bind Ni2+ and Co2+ over other cations from first series transition metals. Conserved structure motifs, comprising about 15 nt, were identified that are likely to represent the Ni2+ binding sites. Two conserved motifs contain an asymmetric purine-rich internal loop and probably a mismatch G-A base pair. The structure of one of these motifs was studied with proton NMR spectroscopy and formation of the G-A pair at the junction of helix and internal loop was demonstrated. Using Ni2+ as a paramagnetic probe, a divalent metal ion binding site near this G-A base pair was identified. Ni2+ ions bound to this motif exert a specific stabilization effect. We propose that small asymmetric purine-rich loops that contain a G-A interaction may represent a divalent metal ion binding site in RNA.