Crystal and molecular structure of r(CGCGAAUUAGCG): an RNA duplex containing two G(anti).A(anti) base pairs

Non-Watson-Crick base pair associations contribute significantly to the stabilization of RNA tertiary structure. The conformation adopted by such pairs appears to be a function of both the sequence and the secondary structure of the RNA molecule. G.A mispairs adopt G(anti).A(anti) configurations in some circumstances, such as the ends of helical regions of rRNAs, but in other circumstances probably adopt an unusual configuration in which the inter-base hydrogen bonds involve functional groups from other bases. We investigated the structure of G.A pairs in a synthetic RNA dodecamer, r(CGCGAAUUAGCG), which forms a duplex containing two such mismatches.

The structure of the RNA duplex was determined by single crystal X-ray diffraction techniques to a resolution in the range 7.0-1.8A, and found to be an A-type helical structure with 10 Watson-Crick pairs and two G.A mispairs. The mispairs adopt the G(anti).A(anti) conformation, held together by two obvious hydrogen bonds. Unlike analogous base pairs seen in a DNA duplex, they do not exhibit a high propeller twist and may therefore be further stabilized by weak, reverse, three-center hydrogen bonds.

G(anti).A(anti) mispairs are held together by two hydrogen of guanine and the N6 and N1 of adenine. If the mispairs do not exhibit high propeller twist they may be further stabilized by inter-base reverse three-centre hydrogen bonds. These interactions, and other hydrogen bonds seen in our study, may be important in modelling the structure of RNA molecules and their interactions with other molecules.