In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or ER synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process is called gene expression.

1. Translation and post
translational modifications
Yash Gupta
Department of Biotechnology Engineering
Institute of Engineering and Technology
Bundelkhand University Jhansi
Molecular Biology

2. Translation
Translation refers to a process of protein
synthesis from mRNA which act as template
The direction of reading mRNA is 5’ to 3’ and
synthesis of peptide is from N (or amino)
terminal to C (or carboxy) terminal
For every amino acid there is specific 3
bases sequence on mRNA called codon
The machinery for translation consists of
ribosomes, tRNAs as adaptors, mRNA as
template and several proteins which
orchestrate all the components
This process make use of three types of
RNAs: mRNA, rRNA, tRNA
Figure1: Machinery of protein translation.
Source: Karp, Gerald, Cell and Molecular Biology, 5th Ed., Wiley, 2008

3. Genetic codes
Genetic code or codon refers to the group of
3 nucleotide on mRNA which code for a
specific amino acid
One codon specify single amino acid but one
amino acid can be coded by multiple codons
Stop codons terminates translation process
when encountered
Stop codon are 3 in number (UAA, UAG, UGA)
Start codon (AUG) initiates the translation and
codes for amino acid methionine
Codons are read starting from start codon then
continuing till the stop codon in 5’ to 3’ direction
The universality of genetic codes ensure that in
every species a codon codes for the same amino
acid
Figure 2: Genetic codes for amino acids
https://jgi.doe.gov/proving-codon-genetic-code-flexibility/

4. Transfer RNA (tRNA)
Transfer RNA serve as adaptors for binding of amino
acid to mRNA
They have unique sequences to identify right amino
acid and align it to correct codon onmRNA
tRNAs are around 70 to 80 nucleotides in length and
have clover-leaf structure
Every tRNA have CCA sequence at 3´ terminus where the
amino acid attaches at ribose sugar of adenosine residue
Anticodon loop identifies the codon on mRNA template and
binds by forming complementary base pairing
Incorporation of amino acid in peptide chain depends
on its attachment to tRNA
Also the codon to anticodon binding specificity is
highly important
Special enzymes called aminoacyl tRNA synthetases
mediates attachment of amino acids totRNA
Figure 3: Structure of tRNA
Source: The Cell: A MolecularApproach. 2nd edition. Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.

5. Mechanism of amino acid attachment to tRNA
Figure 4: Amino acid attachment to tRNA
Source: The Cell: A MolecularApproach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.

6. Ribosomes
Ribosomes contribute to the main machinery for translation
They provide the sites for synthesis of proteins in both
eukaryotic and prokaryotic cells
They are named according to their sedimentation rate in
ultracentrifugation
Ribosome of bacteria is 70S and in eukaryotes is of80S
Each ribosome has two subunits made up of rRNAs and
proteins
Number of ribosomes in E.coli is 20,000 where as in mammals
id 10 million
E.coli ribosome contain small subunit (30S) made up of 21 proteins
and16S rRNA, and large subunit has 34 proteins and 23S and 5S
rRNA
Eukaryotic ribosome contain small subunit (40S) made up of
approx. 30 proteins and 18S rRNA, and the large subunit (60S) is
made up of approx. 45 proteins and 5S, 5.8S and 28S rRNAs
Figure 5: Structure of ribosomes
Source: The Cell: A MolecularApproach. 2nd edition.
Cooper GM. Sunderland (MA): Sinauer Associates; 2000.

7. mRNA
RNA that
DNA to
mRNAs are messenger
conveys message from
proteins
These mRNAs contain regions that are
not translated into protein at their 3’ and
5’ ends called untranslated regions
Prokaryotic mRNA is polycistronic i.e. one
mRNA encodes for multiple proteins
Eukaryotic mRNA is monocistronic i.e. it
codes for multiple proteins and also
contain modifications like m7G (7 methyl
guanosine) cap at 5’ end and poly A tail
at 3’ end
Figure 6: mRNAs of prokaryotic and eukaryotic cells
Source: The Cell: A MolecularApproach. 2nd edition. Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.

8. Translation factors of prokaryotes and
eukaryotes
Role Prokaryotes Eukaryotes
Initiation factors IF-1, IF-2, IF-3 eIF-1, eIF-1A, eIF-2,
eIF-2B, eIF-3, eIF-4A,
eIF-4B, eIF-4E, eIF-
4G, eIF-5
Elongation factors EF-Tu, EF-Ts, EF-G eEF-1α, eEF-1βγ,
eEF-
2
Termination factors RF-1, RF-2, RF-3 eRF-1, eRF-3

9. Specific mRNA components
for translation initiation
Initiation of translation is not random but
requires specific site on the mRNAs
In prokaryotes Shine-Delgarno sequence
which lie upstream of the start codon AUG
initiates the process
Base pairing of SD sequence and 3’ end
of16S rRNA aligns mRNA over ribosome
In eukaryotes 40S subunit of ribosome binds
to the 5’ m7G cap and then scan for AUG
initiation codon to start translation
Figure 7: Translation initiation signals
Source: The Cell: A MolecularApproach. 2nd edition. Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.

10. Process of translation
The process of translation has three
steps 1) Initiation 2) Elongation 3)
Termination
At initiation (of both eukaryotes and
prokaryotes) specific tRNA of
methionyl and mRNA binds to small
subunit of ribosome
After that the large subunit of ribosome
join this complex
This proceeds to elongation step of
polypeptide
Many proteins not associated to
ribosome called translation factors
also take part in this process
Different factors initiates translation
in prokaryotes and eukaryotes
Figure 8: Stages of translation
Source: The Cell: A MolecularApproach. 2nd edition. Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.

11. Initiation in bacteria
IF1, IF2 & IF3 binds 30S subunit of
ribosome
Initiator tRNA (N-formylmethionyl tRNA)
and mRNA joins GTP-bound IF2 and
this releases IF3
Now 50S subunit joins the complex and
triggers GTP hydrolysis to GDP and
lead to release of IF2-GDP and IF1
This release form the initiation
complex ready for elongation stage
Figure 9: Initiation complex in bacteria (Source: The Cell: A
Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA):
Sinauer Associates; 2000.)

12. Initiation in eukaryotes
Initiation in eukaryotes needs 10 proteins
elf-1A, elf-1 and elf-3 binds 40s subunit of ribosome
elf-2 bound to GTP binds with initiator tRNA (methionyl
tRNA)
mRNA is identified and transported to ribosome by group of
elf-4 factors
elf-4E recognizes 5’ cap
elf-4G binds to PABP (poly A binding protein) which is
associated to mRNA 3’ poly A tail
Now elf-4E and elf-4G associate with elf-4A and elf-4B and
bring mRNA to 40S subunit of ribosome, here elf-4G interacts
with elf-3
40S subunit in association with initiator tRNA scans
mRNA for AUG codon
When AUG encounters elf-5 hydrolyze GTP on elf-2 and all
initiation factors get released leaving site for 60S subunit to
bind 40S and form initiation complex of 80S
Figure 10: Initiation complex in eukaryotes (Source: The Cell: A
Molecular Approach. 2nd edition. Cooper GM. Sunderland
(MA): Sinauer Associates; 2000.)

13. Elongation
3 sites of ribosome P, A, E site named as peptidyl, aminoacyl and exit respectively
play major role in elongation
The initiator tRNA binds to P site then next amino acyl tRNA binds to A site by
base-pairing second codon on mRNA
EF-Tu (prokaryotes) or eEF-1a (eukaryotes) bound to GTP escorts amino acyl
tRNA to ribosome
GTP hydrolyses to GDP when exact tRNA inserts at A site and this elongation factor
is released in GDP conjugatedform
This GTP hydrolysis before release of elongation factor is a rate limiting step and
provides time for dissociation if an incorrect tRNA encounters the site (proof reading)
before peptide bondformation
As EF-Tu or eEF-1a leaves, peptide bond forms between initiator tRNA (P site) and
second tRNA (A site) resulting transfer of methionine to second tRNA (forming
peptidyl tRNA)
This reaction is catalyzed by rRNA of large subunit of ribosome
EF-G (prokaryotes) or eEF-2 (eukaryotes) coupled with GTP now performs
translocation in which GTP hydrolysis moves ribosome 3 nucleotide ahead on
mRNA
This positions next codon in A site and peptidyl tRNA from A site to P site and
initiator uncharged tRNA from P site to Esite
Now peptidyl tRNA at P site is left and A site becomes empty for next tRNA as next
tRNA binds at A site uncharged tRNA leaves from Esite
The process goes on until stop codon is encountered
Figure 11: Elongation of translation process (Source:
The Cell: A Molecular Approach. 2nd edition. Cooper
GM. Sunderland (MA): Sinauer Associates; 2000.)

14. Regeneration of elongation factor
During elongation the released EF-Tu or
eEF-1a in GDP bound form needs to be
converted to GTP form
EF-Ts or eEF-1βγ binds EF-Tu or Eef-1a
in GDP bound form and exchange GDP
for GTP
This regenerates EF-Tu-GTP or eEF-1a-
GTP ready to escort tRNA to ribosomal A
site
Figure 12: Elongation factor regeneration (Source: The Cell: A Molecular Approach.
2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.)

15. Termination
Elongation stops as a stop codon (UAA, UAG, UGA)
encounters ribosomal A site
No tRNAs are present for these anticodons
Release factors identifies these codons and bind them thus
terminates synthesis of protein
Prokaryotes release factors: RF-1 identifies UAA/UAG and RF- 2
identifies UAA/UGA
Eukaryotes release factor eRF-1 identifies all stop codons
Another release factor RF-3 (prokaryotes) and eRF-
3
(eukaryotes) act along with the above factors in termination
These release factors bind stop codon at A ribosomal site and
hydrolyze bond in between polypeptide and tRNA present at P site
This releases polypeptide as well as tRNA and dissociates
subunits of ribosomes and mRNA complex
mRNA translation can occur simultaneously by many ribosomes as
one ribosome moves far away from site of initiation another
ribosome can bind initiation site of that mRNA and can synthesize
polypeptide
The mRNA bound to many ribosomes is known as polysome
or polyribosome
Figure 13: Termination of translation (Source: The Cell:
A Molecular Approach. 2nd edition. Cooper GM.
Sunderland (MA): Sinauer Associates; 2000.)

16. Regulation of translation
Repressor protein binding to specific site on
mRNA result in translation inhibition
Regulation of ferritin synthesis in eukaryotes
occur by this mechanism
When iron is in abundance more ferritin is
synthesized
IRE (iron response element) binds
untranslated 5’ region of ferritin mRNA in
absence of iron and blocks translation
Figure 14: Regulation of ferritin synthesis (Source: The Cell: A Molecular
Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer Associates; 2000.)

17. Regulation of regeneration of eIF-2 in
eukaryotes
Protein kinase phosphorylates elf-2 and
blocks bound GDP exchange for GTP
This inhibits translation initiation
Reticulocytes have such regulation for
globin proteins
If heme is present in adequate amount
then translation occurs
If amount of heme is not sufficient then protein
kinase phosphorylate elf-2 inhibits translation
Figure 15: Regulation of regeneration of elf-2 translation factor (Source: The Cell: A
Molecular Approach. 2nd edition. Cooper GM. Sunderland (MA): Sinauer
Associates; 2000.)

18. Inhibitor antibiotics for translation
Antibiotics Target cells
(Prokaryotes(P)/Eukaryotes (E))
Effect on translation
Streptomycin P Initiation inhibitor; cause
misreading
Tetracycline P Aminoacyl tRNAs binding
Inhibition
Chloramphenicol P Peptidyl transferase
inhibitor
Erythromycin P Translocation inhibitor
Puromycin P and E Results in premature
termination of chain
Cycloheximide E Peptidyl transferase
inhibitor

19. Post translational modifications
Various post translational modifications occur on polypeptide to transform it to functional protein
Some common type of post translational modifications are as follows:
Formation of disulfide bonds by oxidation of SH group of cysteine in rough endoplasmic reticulum (ER)
lumen is frequent in secretory proteins and membrane proteins (exoplasmic domain)
PDI (Protein disulfide isomerase) catalyzes the disulfide bond rearrangement increasing protein folding
of membrane and secretory proteins in ER
Other proteins which facilitates folding are lectins, calreticulin, calnexinand peptidyl propyl isomerases
Assemblage of subunits in ER of membrane and secretory proteins
Transportation of properly folded proteins from ER to Golgi complex
Unassembled protein subunits or proteins which are abnormally folded retain in ER as they either form
aggregates or are in permanent bounded form with Hsc-70 or other chaperons of ER.
Misfolded and unassembled proteins are sent to cytosol for degradation by ubiquitination or
proteasomal pathways
Some proteins reside in ER by KDEL sequence at C-terminal or they are retrieved to ER by KDEL
receptors from cis Golgi network
Some other modifications that generally occur in different types of proetins are amidation, acetylation,
carboxylation, hydroxylation, glycosylation, methylation, phosphorylation, nitrosylation, proteolytic cleavage, and
sumoylation

20. References
Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates;
2000. Translation of mRNA. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK9849/
Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H.
Freeman; 2000. Section 17.6, Post-Translational Modifications and Quality Control in the Rough
ER. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21741/
Duan G, Walther D. The roles of post-translational modifications in the context of protein
interaction networks. PLoS Comput Biol. 2015 Feb 18;11(2):e1004049. Duan, G., & Walther, D.
(2015). The roles of post-translational modifications in the context of protein interaction
networks. PLoS computational biology, 11(2), e1004049.
https://doi.org/10.1371/journal.pcbi.1004049
Gerald K. Cell and Molecular Biology. 5th edition. Wiley, 2008