THE STRUCTURE OF AN RNA DODECAMER WITH NONCANONICAL BASE PAIRS DOES CHANGES LITTLE UPON CRYOCOOLING. Kurt F. Schaefer, Cindy L. Barnes, Susan E. Lietzke, and Craig E. Kundrot. Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215

The structure of an RNA dodecamer duplex, r(GGCGCUUGCGUC), has been determined at room temperature (RT) at 2.9 Å resolution and ~100 K at 2.4 Å resolution. The structure contains tandem U-U base-pairs. Previous studies with proteins indicate that the unit cell volumes decrease from 1% to 7% upon cryocooling, and similar results have been observed with cryocooled DNA duplexes. With the exception of one DNA octamer displaying a range of helical conformations, similar DNA structures were observed at room temperature and 100K. The recent use of cryocooling to determine the structures of RNA duplexes, the hammerhead ribozyme and the P4/P6 region of a Group I intron, has prompted us to examine how cryocooling affects the structure of an RNA duplex. The unit cell volume of the dodecamer crystals decreases 7% increase upon cryocooling. But a partially refined model (R=22.2% and R-free =22.9%) shows that the structures have a r.m.s.d. of only 0.2 Å. The results of comparing the fully refined RT structure to the 100 K structure will be presented.