tRNA - The Adapter Molecule

Transfer RNA, tRNA, is the molecule that physically links a codon on an mRNA to the correct amino acid, because a codon's sequence of bases has no chemical affinity of its own for any particular amino acid; tRNA acts, in effect, as a shuttle that picks up a specific amino acid from the pool scattered through the cytoplasm at one end while independently reading and recognising a specific mRNA codon at the other, which is exactly why Francis Crick, who first proposed this role, named it the 'adapter molecule'. The secondary structure of tRNA was worked out by Robert Holley, whose two-dimensional 'clover-leaf' model remains the standard way of representing it: the molecule folds back on itself to form several base-paired stem regions separating single-stranded loops, giving an outline that resembles a four-leaf clover. Although this clover-leaf diagram is the conventional way to draw tRNA, the molecule's actual three-dimensional shape is considerably more compact, folding further into a structure that looks like an inverted letter L. The clover-leaf model identifies three principal arms -- the DHU arm (named for the modified base dihydrouridine it contains), a middle arm, and the T-psi-C arm (named for the modified bases thymidine and pseudouridine it contains) -- each ending in a loop: an amino-acyl binding loop, an anticodon loop, and a ribosomal binding loop, respectively. The model also shows a small additional lump, called the variable loop or extra arm, whose size differs between different tRNA molecules. One end of the folded molecule, the amino-acid acceptor end, is where a specific amino acid becomes covalently attached, while the opposite end presents a set of three unpaired anticodon nucleotides that directly base-pair with the matching codon on an mRNA molecule, ensuring that the correct amino acid, and no other, is delivered into the growing polypeptide chain at that position. Modified bases of the kind found in the DHU and T-psi-C arms are especially characteristic of tRNA, and the pairing that occurs between an anticodon and its codon shows a degree of flexibility, called wobbling, that lets a single tRNA molecule successfully read more than one closely related codon. The process by which an amino acid is attached to its matching tRNA is called aminoacylation, or charging, and the resulting product is referred to as a charged, or aminoacyl-, tRNA; a tRNA that has not yet been joined to its amino acid is correspondingly called an uncharged tRNA. Once two appropriately charged tRNA molecules are brought together at the ribosome, the formation of a peptide bond between their respective amino acids becomes energetically favourable, and it is by repeating this process, one amino acid at a time, that a growing polypeptide chain is progressively assembled. Aminoacylati …