4.2 The structure of tRNA

Transfer RNAs are small and compact molecules. Comparisons of the base sequences of many tRNAs led to the predicted four-leaf clover structure shown in Figure 18a, which follows the rule of maximising base-pairing interactions. This structure was largely confirmed by analysis with single-strand nucleases.

Two of the four main arms of the tRNA molecule are named according to their function, i.e. binding to the mRNA trinucleotide that encodes a specific amino acid (anticodon arm), or to the specific amino acid itself (acceptor arm). The dihydrouridine and TΨC arms are named from the unusual bases that always appear in these locations (see Figure 4b for the structures of some of these bases). When the tertiary structure of the tRNA was determined by X-ray diffraction, it became clear that the cloverleaf structure is, in fact, folded into an L-shaped molecule in three dimensions, shown in Figure 17b. This three-dimensional structure is formed through several tertiary-structure-stabilising interactions, as represented in Figure 17c, which include hydrogen bonds in base triples (three nearby bases linked through hydrogen bonding). A base triple may stabilise tertiary interactions between widely separated parts of a polynucleotide chain, just as non-covalent interactions between amino acid residues stabilise tertiary structure in proteins. Extensive methylation of tRNA confers resistance to cellular nucleases, and tRNA molecules tend to be longer-lived than mRNAs.