THE CRYSTAL-STRUCTURE OF AN RNA OLIGOMER INCORPORATING TANDEM ADENOSINE-INOSINE MISMATCHES

The X-ray crystallographic structure of the RNA duplex [r(CGCAIGCG)](2 ) has been refined to 2.5 Angstrom. It shows a symmetric internal loop of two non-Watson-Crick base pairs which form in the middle of the du plex. The tandem A-I/I-A pairs are related by a crystallographic two-f old axis. Both A(anti)-I(anti) mismatches are in a head-to-head confor mation forming hydrogen bonds using the Watson-Crick positions. The oc tamer duplexes stack above one another in the cell forming a pseudo-in finite helix throughout the crystal. A hydrated calcium ion bridges be tween the 3'-terminal of one molecule and the backbone of another. The tandem A-I mismatches are incorporated with only minor distortion to the backbone. This is in contrast to the large helical perturbations o ften produced by sheared G-A pairs in RNA oligonucleotides.