Solution structure of an unusually stable RNA hairpin, 5'GGAC(UUCG)GUCC.

Hairpin loops are important structural elements of RNA, helping to define the three-dimensional structure of large RNAs and providing potential nucleation sites for RNA folding and interaction with other nucleic acids and proteins. Little, however, is known about the conformation of RNA hairpins, most of what we know coming from transfer RNA crystal structures and from studies of DNA hairpins. We report here the determination of the structure of a very stable and common RNA hairpin, 5'GGAC(UUCG)GUCC (loop nucleotides in parenthesis), by NMR spectroscopy. The sequence C(UUCG)G occurs very often in RNA and may be a nucleation site for RNA folding and a protein-binding site. A high-resolution structure for the hairpin was derived from interproton distances and scalar coupling constants determined by NMR. The loop is stabilized by a G.U base pair, with guanine in the syn conformation, a cytosine-phosphate contact and extensive base stacking. These findings and other structural features of the loop can explain the unusual stability of the hairpin and suggest why reverse transcriptase cannot read through the loop, although it can transcribe through other kinds of RNA secondary structure.