Difference Between Prokaryotic and Eukaryotic Translation

Main Difference – Prokaryotic vs Eukaryotic Translation

Prokaryotic and eukaryotic translation are involved in the synthesis of proteins by decoding the genetic instructions carried by the mRNAs. During translation, nucleotide triplets, known as codons, on the mRNA are translated into a sequence of amino acids. Both prokaryotic and eukaryotic translation share a similar basic plan throughout the processes. However, there are several differences that can be observed in these translation processes. The main difference between prokaryotic and eukaryotic translation is that prokaryotic translation occurs synchronously with its transcription whereas eukaryotic translation occurs asynchronously with its transcription.              

This article explains,

1. What is Prokaryotic Translation
      – Definition, Process, Features
2. What is Prokaryotic Translation
      – Definition, Process, Features
3. What is the difference between Prokaryotic and Eukaryotic Translation

What is Prokaryotic Translation

In prokaryotes, translation is the process of simultaneously synthesizing proteins with transcription. Translation begins just after transcribing the 5′ end of the gene into mRNA. Prokaryotic translation basically occurs in three steps: initiation, elongation and termination. In order to initiate the translation, the two subunits 50S and 30S are assembled. The three initiation factors, IF1, IF2 and IF3 help to assemble the initiation complex. N-formylmethionine is the first added amino acid in translation. GTP is used as the energy source for the peptide bond formation between the remaining and incoming nucleotides. The translation initiation factor is EF-P. 

The selection of the start codon is facilitated by the binding of the ribosome with the Shine-Dalgarno sequence. The Shine-Dalgarno sequence is a purine-rich region located upstream of the AUG start codon. This sequence is complementary to the pyrimidine-rich region on 16S rRNA. The 16S rRNA is a component of 30S subunit. These two complementary nucleotides pair together, forming a double-stranded RNA structure. This pairing brings the initiation codon into the P-site of the ribosome. The first amino acid binds with the P site. A ribosome consists of three active sites: A site, P site and E site. Incoming aminoacyl tRNAs, other than the first aminoacyl tRNA, binds with the A site. The peptide bond formation occurs at the P site. Exit site for the uncharged tRNA is the E site.

Figure 1: Transcription initiation in prokaryotes’ 70S ribosome

The two elongation factors are EF-G and EF-Tu. The translation elongates until the ribosome reaches to one of the three stop codons: UAA, UGA, UAG. Release factors other than the tRNAs, recognise the stop codon. The mRNA with the termination codon at A site is referred to as the termination complex. Three release factors can be identified: RF1, RF2 and the RF3. RF1 and RF2 recognise the UAA/UAG and UAA/UGA and hydrolyze the ester bond in peptidyl-tRNA to release the nascent polypeptide chain. RF3 catalyses the releasing of RF1 and RF2. Once the new protein releases, the ribosome undergoes recycling. The Ribosome Recycling Factor and EF-G, are involved in releasing mRNA and tRNAs from the ribosome and dissociation of 70S ribosome into 30S and 50S subunits. IF3 releases the mRNA by replacing the deacylated tRNA.

When bacteria enter the stationary phase, the translation is downregulated by the dimerization of ribosomes. Ribosome dimerization is facilitated by RMF, HPF and YfiA. The ribosome-dissociation factors are RsfA and HflX.

What is Eukaryotic Translation

Translation is the second step of eukaryotic gene expression, a separate event from eukaryotic transcription. Transcription and translation occur in two different compartments in eukaryotes. Therefore, the two processes can not occur simultaneously. Eukaryotic mRNAs are monocistronic and are processed in the nucleus by adding a 5′ cap, poly A tail and splicing out of introns before they are released to the cytoplasm. Ribosomal pausing also affects the translation by co-translational folding of the newly synthesising polypeptide chain on the ribosome. This process delays translation, giving time for the translation.  

Eukaryotic mRNAs consist of a 5′ cap and poly A tail. Therefore, the initiation of translation occurs in two different ways: cap-dependent initiation and cap-independent initiation. During cap-dependent initiation, the initiation factors bind to the 5′ end of the mRNA. These initiation factors hold the mRNA in the small subunit of the ribosome. During cap-independent initiation, internal ribosome entry sites allow the ribosome trafficking to the start site by direct binding. In eukaryotes, the first binding amino acid is methionine. 40S subunit associates with 60S subunit to form 80S ribosome.

Two elongation factors are involved in eukaryotic translation: eEF-1 and eEF-2. Elongation occurs in a similar way to that of prokaryotes. Termination of the translation is also the same as in the prokaryotic system. But the universal release factor eRF1 is capable of recognising all three stop codons. The release factor, eRF3 helps eRF1 to release the polypeptide chain. Basic steps of the translation are shown in figure 2.

Figure 2: Generalised translation   

Difference Between Prokaryotic and Eukaryotic Translation

Timing

Prokaryotic Translation: Prokaryotic transcription and translation are simultaneous processes.

Eukaryotic Translation: Eukaryotic transcription and translation are discontinuous processes.

Ribosomes

Prokaryotic Translation: 30S and 50S = 70S ribosomes

Eukaryotic Translation: 40S and 60S = 80S ribosomes

Messenger RNA Source

Prokaryotic Translation: Porkaryotic mRNAs occur in the cytoplasm. The mRNA is polycistronic.

Eukaryotic Translation: Eukaryotic mRNAs occur in the nucleus. After the post-transcriptional modifications they are released to the cytoplasm via nuclear pores. The mRNA is monocistranic.

Life Span of mRNA

Prokaryotic Translation: The mRNAs are unstable and live for few seconds to two minutes.

Eukaryotic Translation: The mRNAs are quite stable, and live for about few hours to days. 

Location

Prokaryotic Translation: This is performed by 70S ribosomes in the cytoplasm.

Eukaryotic Translation: This is performed by the 80S ribosomes attached with the ER.

Localization in the Cell Cycle

Prokaryotic Translation: There is no definite phase for the occurrence.

Eukaryotic Translation: This occurs in G1 and G2 phases in the cell cycle.

Sequences in the Untranslated Region 

Prokaryotic Translation: Shine-Dalgarno sequence is found in the 5′ UTR, ~10 nucleotides upstream to the start codon.

Eukaryotic Translation: Kozak sequence is found in the 5′ UTR, a few nucleotides upstream to the stat codon.

Translation Initiation

Prokaryotic Translation: Cap- independent initiation.

Eukaryotic Translation: Both cap-dependent and cap-independent initiation.

Initiation Factors

Prokaryotic Translation: Three initiation factors are involved: IF1, IF2 and IF3.

Eukaryotic Translation: Nine initiation factors are involved: elF 1, 2, 3, 4A, 4B, 4C, 4D, 5 and 6.

First Amino Acid

Prokaryotic Translation: N-formylmethionine is the first amino acid added to the polypeptide chain.

Eukaryotic Translation: Methionine is the first amino acid added to the polypeptide chain.

Elongation Factor

Prokaryotic Translation: Two elongation factors are involved: EF-G and EF-Tu.

Eukaryotic Translation: Two elongation factors are involved: eEF-1 and eEF-2.

Speed

Prokaryotic Translation: Prokaryotic translation is a faster process which adds twenty amino acids per second.

Eukaryotic Translation: Eukaryotic translation is a slower process which adds a single amino acid per second.

Fate of First Amino Acid

Prokaryotic Translation: The formyl group is removed from first amino acid, retaining methionine in the polypetide chain.

Eukaryotic Translation: The whole methionine is removed from the polypeptide chain.

Release Factor

Prokaryotic Translation: Two released factors are involved: RF1 (for UAG and UAA) and RF2 (for UAA and UGA).

Eukaryotic Translation: A single release factor is involved: eRF1.

Conclusion

Translation is the universal process of synthesizing proteins as the second step in gene expression. Both prokaryotic and eukaryotic ribosomes decode mRNAs in fundamentally similar methods. Ribosomes are the machinery of the protein synthesis. All twenty essential amino acids are shared in both prokaryotic and eukaryotic translation processes. Both processes occur in the cytoplasm, completing the four processes: initiation, elongation, translocation and termination. The tRNA, brings the correct amino acid, allowing peptide bonds to form between two amino acids. The main difference between prokaryotic and eukaryotic translation is that prokaryotic translation is a simultaneous process with transcription whereas eukaryotic translation is a separate process from its transcription.

Reference:
1. “Prokaryotic translation”. Wikipedia, the free encyclopedia, 2016. Accessed 26 Feb 2017
2. “Eukaryotic translation”. Wikipedia, the free encyclopedia, 2016. Accessed 26 Feb 2017
3. “Difference Between Prokaryotic and Eukaryotic Translation”. EASY BIOLOGY CLASS, 2017. Accessed 26 Feb 2017
4. Berg JM, Tymoczko JL, Stryer L “Eukaryotic protein synthesis Differs from Prokaryotic protein synthesis Primarily in Translation Initiation”. Biochemistry. 5th edition. Section 29.5, 2002 New York: W H Freeman, New York. NCBI Bookshelf. Accessed 26 Feb 2017

Image Courtesy:
1. “121-70SRibosomes initiation”By David Goodsell – RCSB PDB Molecule of the Month (CC BY 3.0) via Commons Wikimedia
2. “TRNA ribosomes diagram en” By LadyofHats – Own work (Public Domain) via Commons Wikimedia