GNRA tetraloops make a U-turn.

The U-turn (uridine turn) is an RNA structural motif that contains a change in backbone direction stabilized by specific interactions across the bend. It was first identified in the anticodon loop and the T-loop of yeast tRNA(Phe) (Quigley & Rich, 1976, Science 194:796-806) and has recently also been found in the crystal structure of the hammerhead ribozyme (Pley HW, Flaherty KM, McKay DB, 1994a, Nature 372:68-74). These U-turn motifs follow a UNR consensus sequence (where N is any nucleotide and R is G or A). Here we report that the frequently occurring GNRA tetraloops also contain a U-turn motif, and we discuss the role of U-turns as abundant tertiary structural motifs in RNA.