The main objectives of this research are: a) crystallization studies of ribosomal RNAs and their hydrolytic fragments, b) determination of the structure of E. coli Arg tRNA at 4 Angstrom units, c) binding of small molecules to phe tRNA, d) completion of refinement studies on monoclinic yeast phe tRNA, and e) conformational perturbations in tRNA. We have commenced some work on the crystallization of 5S rRNA and plans are underway to perform crystallization studies on the more complex rRNAs and their hydrolytic fragments. 4 Angstrom units data have been collected on E. coli Arg tRNA containing two independent molecules. The orientation and position of these in the unit cell has been obtained by molecular replacement techniques. Fourier techniques are now being used to refine the positions. We have already reported our results of the single-site specific binding of the ethidium dye to phe tRNA (PNAS 74, 4821 (1977)). The dye binds in a pocket in the P10 loop of the tRNA. We have found that proflavin also binds in a nonintercalative manner to the tRNA. The binding is specifically to the backbone phosphates at three regions of the molecule. Similar studies are planned for other dyes, drugs and mutagens. In continuation of our refinement studies of monoclinic phe tRNA at 2.5 Angstrom units, we are in the process of inclusion and sorting out the solvent and cation binding sites. These structural studies continue to advance our understanding of the structures, conformations, and interactions of the ribonucleic acids.