The objective of this project is to determine the three dimensional structure and physical properties of several commonly occurring RNA secondary structure motifs. Among the motifs to be examined are helices containing non-Watson Crick base pairs, and bulged bases, the loops of hairpin structures, and pseudoknots. High field NMR and x-ray crystallography will be the tools used. The structures in question are important components of all naturally occurring RNAs. RNAs serve essential roles as both substrates and catalysts in the gene expression pathway. The information gained should help us understand how these molecules perform their critical biological functions. The systems to be studied are: (1) the helix IV-V region of the 5S RNA from E. coli, which poses interesting questions about base mismatches and protein binding, (2) the pseudoknot structure at the 5' end of U2 RNA, an RNA essential for splicing reactions in eukaryotes, and (3) several purely synthetic RNAs that have simple loops sequences and secondary structure irregularities.