A curved RNA helix incorporating an internal loop with G center dot A and A center dot A non-Watson-Crick base pairing

The crystal structure of the RNA dodecamer 5'-GGCC(GAAA)GGCC-3' has been determined from x-ray diffraction data to 2.3-Angstrom resolution. In the crystal, these oligomers form double helices around twofold symmetry axes. Four consecutive non-Watson-Crick base pairs make up an internal loop in the middle of the duplex, including sheared G . A pairs and novel asymmetric A . A pairs. This internal loop sequence produces a significant curvature and narrowing of the double helix. The helix is curved by 34 degrees from end to end and the diameter is narrowed by 24% in the internal loop. A Mn2+ ion is bound directly to the N7 of the first guanine in the Watson-Crick region following the internal loop and the phosphate of the preceding residue. This Mn2+ location corresponds to a metal binding site observed in the hammerhead catalytic RNA.