Solution structure of the CUUG hairpin loop: a novel RNA tetraloop motif.

The solution structure of a uniformly 13C/15N-labeled CUUG RNA hairpin loop has been determined by multidimensional heteronuclear magnetic resonance spectroscopy in combination with distance geometry and restrained molecular dynamics calculations. The structure of this CUUG tetraloop represents a novel RNA loop motif where the first and last loop nucleotides form a standard Watson-Crick C-G base pair and the second loop nucleotide interacts directly with the closing base pair of the stem by folding into the minor groove. This structure helps explain why the closing base pair is phylogenetically conserved and indicates a six-nucleotide G(CUNG)C motif for the CUUG RNA tetraloop. Implications for the function of this CUUG tetraloop in ribosomal RNA and in RNA tertiary interactions are discussed.