Transfer RNA serves as a link between the genetic code in mRNA and the amino acids that make up a protein. Each tRNA attaches to a particular amino acid and carries it to the ribosome, where the tRNA adds its amino acid to the growing polypeptide chain at the position specified by the genetic instructions in the mRNA.

THE STRUCTURE OF TRANSFER RNA Each tRNA is capable of attaching to only one type of amino acid. The complex of tRNA plus its amino acid can be written in abbreviated form by adding a three-letter superscript representing the amino acid to the term “tRNA.” For example, a tRNA that attaches to the amino acid alanine is written as tRNA^Ala.

A unique feature of tRNA is the presence of rare modified bases. All classes of RNAs have nucleotides containing the four standard bases (adenine, cytosine, guanine, and uracil) specified by DNA, but tRNAs have nucleotides containing additional bases, including ribothymine, pseudouridine (which is also occasionally present in snRNAs and rRNA), and dozens of others.

All tRNAs are short RNA molecules that are similar in their secondary structure, a feature that is critical to tRNA function. Some of the nucleotides in a tRNA are complementary to each other and form intramolecular hydrogen bonds. As a result, each tRNA has a cloverleaf structure (Figure 10.24). All tRNAs have the same sequence (CCA) at the 3′ end, where the amino acid attaches to the tRNA. On each tRNA is a set of three nucleotides that make up the anticodon, which pairs with the corresponding codon on the mRNA during protein synthesis to ensure that the amino acids link in the correct order.

Although each tRNA molecule folds into a cloverleaf owing to the complementary pairing of bases, the cloverleaf is not the three-dimensional (tertiary) structure of tRNAs found in the cell. The results of X-ray crystallographic studies have shown that the cloverleaf folds on itself to form an L-shaped structure, as illustrated by the space-filling and ribbon models in Figure 10.24.

TRANSFER RNA PROCESSING Both bacterial and eukaryotic tRNAs are extensively modified after transcription. In E. coli, several tRNAs are usually transcribed together as one large precursor tRNA, which is then cut up into pieces, each containing a single tRNA. Additional nucleotides may then be removed one at a time from the 5′ and 3′ ends of the tRNA in a process known as trimming. Base-modifying enzymes may then change some of the standard bases into modified bases. In some prokaryotes, the CCA sequence found at the 3′ end of each tRNA is encoded in the tRNA gene and is transcribed into the tRNA; in other prokaryotes and in eukaryotes, this sequence is added by a special enzyme that adds the nucleotides without the use of any template. Eukaryotic tRNAs are processed in a manner similar to bacterial tRNAs: most are transcribed as part of larger precursors that are then cleaved, trimmed, and modified to produce mature tRNAs.