Translation in Eukaryotes
Eukaryotic translation is the biological process through which messenger RNA is translated into proteins in eukaryotes. It consists of four phases: initiation, elongation, termination, and recycling.
Eukaryotic translation is satisfyingly similar to the prokaryotic case. The initiation process is slightly more complicated, but the elongation and termination processes are the same, but with eukaryotic homologues of the appropriate elongation and release factors.
- The first step is the formation of a pre-initiation complex consisting of the 40S small ribosomal subunit, Met-tRNA, eIF-2 and GTP.
- The pre-initiation complex binds to the 5’ end of the eukaryotic mRNA, a step that requires eIF-4F and eIF-3.
- The eIF-4F complex consists of eIF-4A, eIF-4E, and eIF-4G; eIF-4E binds to the 5’ cap on the mRNA whilst eIF-4G interacts with the poly (A) binding protein on the poly (A) tail.
- The eIF-4A is an ATP-dependent RNA helicase that unwinds any secondary structures in the mRNA, preparing it for translation.
- The complex then moves along the mRNA in a 5’ to 3’ direction until it locates the AUG initiation codon.
- The 5’ untranslated regions of eukaryotic mRNAs vary in length but can be many hundred nucleotides long and may contain secondary structures such as hairpin loops. These secondary structures are probably removed by initiation factors of the scanning complex.
- The initiation codon is usually recognizable because it is often contained in a short sequence called the Kozak consensus (5’-ACCAUGG-3’).
- Once the complex is positioned over the initiation codon, the 60S large ribosomal subunit binds to form an 80S initiation complex, a step that requires the hydrolysis of GTP and leads to the release of several initiation factors.
- Elongation depends on eukaryotic elongation factors.
- Three elongation factors, eEF-1A, eEF-IB and eEF-2, are involved which have similar functions to their prokaryotic counterparts EF-Tu, EF-Ts and EF-G.
- At the end of the initiation step, the mRNA is positioned so that the next codon can be translated during the elongation stage of protein synthesis.
- The initiator tRNA occupies the P site in the ribosome, and the A site is ready to receive an aminoacyl-tRNA.
- During chain elongation, each additional amino acid is added to the nascent polypeptide chain in a three-step microcycle.
- The steps in this microcycle are:
- Positioning the correct aminoacyl-tRNA in the A site of the ribosome,
- Forming the peptide bond and
- Shifting the mRNA by one codon relative to the ribosome.
- Although most codons encode the same amino acids in both prokaryotes and eukaryotes, the mRNAs synthesized within the organelles of some eukaryotes use a variant of the genetic code.
- During elongation in bacteria, the deacylated tRNA in the P site moves to the E site prior to leaving the ribosome. In contrast, although the situation is still not completely clear, in eukaryotes the deacylated tRNA appears to be ejected directly from the ribosome.
- Termination of elongation depends on eukaryotic release factors.
- In eukaryotes, eukaryotic release factor eRF-1 recognizes all three termination codons (UAA, UAG and UGA) and, with the help of protein eRF-3, terminates translation.
- Upon termination, the ribosome is disassembled and the completed polypeptide is released.
The ribosome is still bound to the mRNA, after the release of the polypeptide and release factors, and is left with two deacylated tRNA. To take part in a new round of polypeptide synthesis, these mRNA and tRNA must be released and the ribosome must dissociate into small and large subunits.