This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Trypanosomes have a unique RNA editing system in their mitochondrion that involves template-directed insertion and deletion of large numbers of uridines to yield functional transcripts from nonfunctional precursor mRNAs. This unique RNA editing system is essential for the survival of these human parasites, so it is a potential drug target. We are interested in the structural details of the duplexes formed between the RNAs that direct the editing (guide RNAs or gRNAs) and their cognate messenger RNAs. This duplex is found in the middle of a RNA editing machine (the editosome). The duplex has three domains. One domain consists 10-15 contiguous Us at the 3? end of the gRNA. This polyU tail binds the mRNA and stabilizes the gRNA-mRNA duplex during the editing reaction. Little is known about the structure of this domain, and there are no examples of RNA structures with such a long run of Us. We have crystals of several variants of the polyU domain. A crystal of one variant diffracts to 2.8 [unreadable] at 100 K with in-house x-rays. We want to do a Br-MAD phasing experiment followed by several monochromatic x-ray diffraction experiments to obtain accurate structures to discover the structure of the polyU domain and its implications for the structure of the editosome.