Several research groups initiated programs to determine tRNA's tertiary structure in the late 1960s. Progress was quite slow at first because of the difficulty in obtaining crystals that were satisfactory for x-ray diffraction analysis. Finally, in 1974 two independent research groups, one led by Alexander Rich and the other by Aaron Klug, obtained crystals of yeast tRNAphe that were suitable for x-ray diffraction analysis. The crystal structure showed that the cloverleaf folds into an L-shape. The D and anticodon arms stack to form one section of the L, while the acceptor and Т?С arms ?tасk to form the other section.
The tRNA molecule's tertiary structure is stabilized by complex interactions. Helical regions in the acceptor, anticodon, D, and Т?С arms are usually stabilized by Watson-Crick base pairing as well as by non-Watson- -Crick base pairing such as the G : U pair in the tRNAphe acceptor arm. Non-helical regions of the tRNA molecule are stabilized by hydrogenbonding interactions between two or three bases that are not usually considered to be complementary to one another and by hydrogenbonding interactions that involve the 2'-hydroxyl group in ribose. 2'-Hydroxyl interactions are especially interesting because they cannot occur in DNA molecules. Different tRNAs have a similar folding pattern, ensuring that various components of the protein synthetic machinery will be able to recognize the tRNA after an amino acid has been attached to it. However, tRNAs also must have unique features that can be recognized by aminoacyl-tRNA synthetases.
