STRUCTURE OF AN RNA DOUBLE HELIX INCLUDING URACIL-URACIL BASE-PAIRS IN AN INTERNAL LOOP

The crystal structure of the RNA dodecamer 5'-GGACUUUGGUCC-3' has been determined from X-ray diffraction data to 2.6 Angstrom resolution. Th is oligomer forms an asymmetric double helix in the crystal. Four cons ecutive non-Watson-Crick base-pairs are formed in the middle of the du plex including the first intrahelical U-U (or T-T) pairs observed in a n oligonucleotide crystal structure. Two different conformations of U- U pairs are observed in the context of the surrounding sequence. One o f these pairs is highly twisted, allowing a bound water to bridge acro ss strands in the major groove. The crystal packing illustrates a new form of RNA helix-helix interaction.