The key actors involved in translation are the ribosome, which consists of two subunits, the messenger RNA (mRNA), several transfer RNAs (tRNA) as well as numerous other protein factors. Translation proceeds in three phases: initiation, elongation, and termination (all describing the growth of the amino acid chain, or polypeptide that is the product of translation).
Transfer RNA (tRNA) is a small RNA chain (74-93 nucleotides), which is responsible for carrying amino acids in to the prokaryotic ribosome, which in turn links them together to form a protein. It has sites for amino-acid attachment and codon (a particular sequence of 3 bases) recognition. The codon recognition is different for each tRNA and is determined by the anticodon region, which contains the complementary bases to the ones encountered on the mRNA. Each tRNA molecule binds only one type of amino acid, but because the genetic code is degenerate, more than one codon exists for each amino acid.
Amino acids must be attached to the tRNA. This is achieved by enzymes termed amino acyl tRNA synthetases. There exists a different synthetase for each amino acid. This enzyme catalyses the binding of the amino acid with ATP. PP is released to form an amino acid-AMP complex. A tRNA molecule is then substituted for the AMP to form an activated tRNA-amino acid molecule. The binding of ATP gives energy to the molecule which is used later on in the energy intensive elongation phase of translation.
The ribosome used in bacterial protein synthesis consists of a 50S and a 30S subunit, which are made up of protein and ribosomal RNA (rRNA). In other organisms the sizes of these subunits are slightly different, but there is always a large and a small subunit that play similar roles. A ribosome can simultaneously hold three tRNA molecules, one each in the aminoacyl (A), peptidyl (P), and exit (E) sites.
Initiation of translation involves the small ribosomal subunit binding to the 'start' codon on the mRNA, which indicates where the mRNA starts coding for the protein. This codon is most commonly an AUG, but alternative start codons are common in prokaryotes. In eukaryotes and archaea, the amino acid encoded by the start codon is methionine. In bacteria, the protein starts instead with the modified amino acid N-formyl methionine (f-Met). In f-Met, the amino group has been blocked by a formyl group to form an amide, so this amino group cannot form a peptide bond –. This is not a problem because the f-Met it at the amino terminus of the protein. In prokaryotes the binding of the small subunit to the correct place on the mRNA is facilitated by base pairing to a series of bases known as the Shine-Dalgarno sequence, located 8-13 nucleotides before the start site.
The initiator tRNA, carrying Met or f-Met, base pairs to the start codon and sits in the P site of the ribosome. The large subunit then forms a complex with the small and elongation proceeds. A new activated tRNA enters the A site of the ribosome and base pairs with the mRNA. The enzyme peptidyl transferase forms a peptide bond between the adjacent amino acids. As this happens, the amino acid on the P site leaves its tRNA and joins the tRNA at the A site. The ribosome them moves in relation to the mRNA shifting the tRNA at the A site on to the P whilst releasing the empty tRNA, this process is known as translocation.
This procedure repeats until the ribosome encounters one of three possible stop codons, where termination occurs. This stalls protein growth, and release factors, proteins which mimic tRNA, enter the A site and release the protein in to the cytoplasm.
Synthesis of proteins can take place extremely quickly. This is aided by multiple ribosomes being able to attach themselves to one mRNA chain, thus allowing multiple proteins to be constructed at once. An mRNA chain with multiple ribosomes is called a polysome. Also, as prokaryotes have no nucleus, an mRNA can be translated while it is still being transcribed. This is not possible in eukaryotes as translation occurs in the cytoplasm, whereas transcription occurs in the nucleus.