Structure of transfer RNAs: similarity and variability.

Transfer RNAs (tRNAs) are ancient molecules whose origin goes back to the beginning of life on Earth. Key partners in the ribosome-translation machinery, tRNAs read genetic information on messenger RNA and deliver codon specified amino acids attached to their distal 3'-extremity for peptide bond synthesis on the ribosome. In addition to this universal function, tRNAs participate in a wealth of other biological processes and undergo intricate maturation events. Our understanding of tRNA biology has been mainly phenomenological, but ongoing progress in structural biology is giving a robust physico-chemical basis that explains many facets of tRNA functions. Advanced sequence analysis of tRNA genes and their RNA transcripts have uncovered rules that underly tRNA 2D folding and 3D L-shaped architecture, as well as provided clues about their evolution. The increasing number of X-ray structures of free, protein- and ribosome-bound tRNA, reveal structural details accounting for the identity of the 22 tRNA families (one for each proteinogenic amino acid) and for the multifunctionality of a given family. Importantly, the structural role of post-transcriptional tRNA modifications is being deciphered. On the other hand, the plasticity of tRNA structure during function has been illustrated using a variety of technical approaches that allow dynamical insights. The large range of structural properties not only allows tRNAs to be the key actors of translation, but also sustain a diversity of unrelated functions from which only a few have already been pinpointed. Many surprises can still be expected.