Molecular Biology: Translation

Transfer RNA

In The Genetic Code, we explained how each codon in messenger RNA (mRNA) codes for a specific amino acid, and that in the process of translation the mRNA brings the amino acids together to form proteins. That explanation is correct, but it is also simplified, and overlooks a crucial component of the translation process. That component is transfer RNA (tRNA), which acts as a kind of link between the information encoded in the mRNA and the amino acids. If the mRNA is a code, then the tRNA is the key that interprets that code into physical proteins.

This section will describe the structure of tRNA and describe how tRNA can "carry" amino acids; knowledge of these aspects of tRNA will be vital for understanding the actual process of proteins synthesis covered next section.

The Structure of tRNA

Transfer RNA molecules vary in length between 60 and 95 nucleotides, with the majority measuring about 75 nucleotides (much smaller than the normal mRNA strand). Regions of self-complementarity within tRNA creates a cloverleaf- shaped structure.

The cloverleaf, being a cloverleaf, is comprised of three characteristic loops. In the figure above, the loop closest to the 5' end is called the dihydrouridine arm (D arm), because it contains dihydrouridine bases, which are unusual nucleotides common only to tRNA. The loop closest to the 3' end is called the T arm, after its sequence of thymine-pseudouridine-cytosine (pseudouridine is also an unusual base). The loop on the bottom of the cloverleaf contains the anticodon, which binds complementarily to the mRNA codon. Because anticodons bind with codons in antiparallel fashion, they are written from the 5' end to 3' end, the inverse of codons. For example, the anticodon in the figure above should be written 3'-CGU-5'. At the 3' end of the tRNA molecule, opposite the anticodon, extends a three nucleotide acceptor site that includes a free -OH group. A specific tRNA binds to a specific amino acid through its acceptor stem.

The cloverleaf structure shown above is actually a two dimensional simplification of the actual tRNA structure. The cloverleaf is therefore called a secondary structure. In reality, the cloverleaf folds further into a tertiary structure, a sort of vague L-shape. At one end of the L lies the anticodon; at the other is the acceptor stem. The L-shaped structure simply amplifies the two active ends of tRNA: the anticodon and the acceptor stem.

The Wobble Hypothesis

The structure of the anticodon of tRNA helps to explain the degeneracy of the genetic code. Previously, in the SparkNote on the Genetic Code, we saw that more than one codon could specify a particular amino acid. However, now we know that tRNA acts as a go between for the codons of mRNA and amino acids. Each tRNA binds to a specific amino acid, but the anticodons of some tRNA molecules can bind to two or three different codons.

The flexibility of some anticodons is the result of the fact that the 3' end of the anticodon is more spatially confined than the 5' end. As a result, the 5' end of the anticodon is free to hydrogen bond with several base groups located at the 3' position of the codon. This idea is called the wobble hypothesis and has been confirmed by X-ray studies that show that while the 3' and middle positions are held tightly in a specific orientation by stacking interactions, the 5' position is not. The 5' position is called the wobble position because it can move around to allow for its pairing with different bases.

The wobble hypothesis means that if the first and second positions are the same, certain different bases in the third position will code for the same amino acid. Codons that specify for the same amino acid are called "synonyms."

Loading Proteins onto tRNA

Before translation can occur, a molecule of tRNA must be bound with the appropriate amino acid. This two-step process is called "charging". In the first step, called adenylylation, an ATP molecule is hydrolyzed, releasing two phosphates, and, transfigured into AMP, forms a high-energy bond with an amino acid.

In the second step, the amino acid-AMP complex is bound to its specific tRNA molecule by an enzyme to form aminoacyl tRNA. There are twenty distinct enzymes engaged in the formation of aminoacyl tRNA, one for each amino acid. In the process of binding the tRNA and amino acid, the AMP is separated from the amino acid.

Specificity of Charging

The specificity of the charging reaction is maintained through two mechanisms. First, amino acids recognize the correct tRNA through distinguishing features on the tRNA, such as the acceptor stem, D stem, and anti-codon stem. Second, the entire process of charging is goverened by a proofreading system that ensures the appropriate amino acid has been loaded onto each tRNA molecule. The proof- reading mechanism checks the reaction at both steps to ensure proper pairing.