prokaryotic translation mechinry

“Prokaryotic Translation Machinery”
By
KAUSHAL KUMAR SAHU
Assistant Professor (Ad Hoc)
Department of Biotechnology
Govt. Digvijay Autonomous P. G. College
Raj-Nandgaon ( C. G. )

Contents
•Introduction
•Definition
•Factors required for Translation
•Formation of aminoacyl t-RNA
1)Activation of amino acid
2) Transfer of amino acid to t-RNA
•Translation involves following steps:-
1)Initiation
2)Elongation
3)Termination
•Conclusion
•Reference

Introduction
Translation is a process in which the genetic
information present in m-RNA in the form of triplet
codes (codons) directs the sequence of aminoacids in
protein.
In other word Translation is process of Protein synthesis
on mRNA template.

Definition
Translation can be defined as the process in which m-
RNA is utilised for actual assembly of amino acids in the
form of polypeptides through peptide linkages. This
process involves no. of enzymes ,mainly consist of t-RNA
& ribosomes.

Factors involving in Translation
Raw materials for Protein synthesis :
1.Ribosomes
2.Amino acids
3.mRNA
4.tRNA
5.Enzymes : (a) Amino-acid activating enzymes
(amino acyl-tRNA synthetase)
(b) Peptide polymerase system
6. Adenosine triphosphate (ATP) as an energy source
7. Gaunosine triphosphate (GTP) for synthesis of peptide bond

Ribosome
The functionally active ribosome are the centers or factories for
protein synthesis.
Prokaryotic Ribosome – 70S ribosome.
The prokaryotic ribosome is made up of two subunits 30S and 50S
subunit.
 Ribosome contains about 65% of RNA and 35% of protein.
Each ribosome consists of two subunits – one big or one small.

Ribosome
Both subunit consists of one or more molecules of rRNA & an
ribosomal proteins.
When the two subunits are associated with each other in a single
ribosome, the structure is sometime called a monosome.
Functional sites of Ribosome
The functional ribosome has two sites – A site
P site
E site

Each site covers both the subunits. A site for binding of
aminoacyl tRNA and P site is for binding peptidyl tRNA
during the course of translation & E site is exit site.

mRNA
In the process of translation cells use to synthesize polypeptides,
using the codon sequence on the mRNA as a template.
Each sequence on the mRNA strand has its own ribosome
binding site, start and stop codons.

tRNA
Transfer RNA (tRNA) is a sequence of ~75 ribonucleotides that
fold to form a three-dimensional structure.
Each tRNA has an acceptor stem that forms a temporary bond with
one amino acid and an anticodon that can bind to its
complementary codon on mRNA.
They are made up to 70-90 nucleotides and their molecular
weight ranges from 25000 to 30000.
They are small, single stranded molecules which are folded into
two-dimensional clover leaf shape.

The clover leaf construction of t-RNA consists of three arms with a
fourth extra arm occurring frequently.

Various protein factors involved during translation
Process Factor Role
Initiation of
Translation
IF 1
IF 2
IF 3
Stabilizes 30s subunit.
Binds f met – tRNA to 30s – mRNA complex,
binds to GTP & stimulates hydrolysis.
Binds 30s subunit tomRNA.
Elongation of
Polypeptide
EF-TU
EF-TS
EF-G
Binds GTP, brings amino acyl tRNA to the A site
of Ribosome
Generates active EF-TU
Stimulates translocation ; GTP – dependent.
Termination of
translocation &
release of
Polypeptide
RF-1
RF-2
RF-3
Catalyzes release of the Polypeptide chain from
tRNA and dissociation of the transloccation
complex; specific for UAA & UAG termination
codon.
Behave like RF-1 specific for UGA & UAA codon.
Stimulates RF-1 And Rf-2.

Prokaryotic Translation
It occurs in four stages, they are
i) Activation of aminoacids
ii) Initiation of polypeptide synthesis
iii) Elongation
iv) Termination

ACTIVATION OF AMINO ACID
Amino acids in the cytoplasm occur in the inactive form and they
can not take part in protein synthesis. Hence these are activated by
giving them energy. The activation is facillate through ATP, whose
molecules unite with the amino acids forming highly reactive
amino acid phosphate-adenyl complexes which are known as
aminoacyl adenylates. The process of activation is governed by
specific enzyme aminoacyl tRNA synthesis.
AA activating enzyme
AA+ATP AMP AA+PP
Where, AA represents amino acid
ATP represents adenosine triphosphate
AMP represents Adanosine monophosphate
PP represents pyrophosphate (Inorganic)

TRANSFER OFAMINO ACID TO t-RNA
The enzyme bound activated amino acids aminoacyl adenylates
become attached to 3’ end of their respective tRNA molecule.
The attachment is catalysed by the same enzyme amino acyl
transfer RNA synthetase, that catalyse activation of their amino acid.
The product thus formed is known as aminoacyl transfer RNA
complex (amino acyl tRNA).
AA AMP+ tRNA AA- tRNA +Amp
The enzyme bound activated amino acids aminoacyl adenylates
become attached to 3’ end of their respective tRNA molecule.
The attachment is catalysed by the same enzyme amino acyl
transfer RNA synthetase, that catalyse activation of their amino acid.
The product thus formed is known as aminoacyl transfer RNA
complex (amino acyl tRNA).

INITIATION
Step 1-
Translation begins with the association ofnit the 30s ribosomal
subunit with the mRNA at AUG initiation codon.
Step 2-
The formylmethionyl-tRNAfMet becomes associated with the
mRNA and the 30S ribosomal subunit complex by binding to
IF2-GTP.
Step 3-
The 50S subunit joins the complex,GTP is hydrolyzed, and
IF2-GDP is released.
The initiator tRNA enters the P site of the ribosome, whereas all
subsequent tRNAs enter the A site

ELONGATION
Elongation of polypeptide chain is brought about by the regular
addition of amino acids.
At 37°C about 15 amino acids per sec are added to a growing
polypeptide chain , so that in 20 sec a protein with 300 amino acids
can be synthesized.
Elongation a cyclic process involving certain elongation factors
EF-Tu, EF-G, may be divided into three steps.
1.Binding of aminoacyl tRNA to A-site.
2.Peptide bond formation
3.Translocation

Binding of aminoacyl tRNA to A-site.
In this step , an aminoacyltRNA whose anticodon is
complementary to the second codon of the mRNA enters the empty
A site of the ribosome. The binding of the tRNA is accompanied by
the release of EF-Tu-GDP.

Peptide bond formation
 In Peptide bond formation the new peptide bond is formed
between the amino acids whose t-RNAare located in A and P sites
on the ribosome.
The peptide bond formation is catalysed by the peptidyl
transferase.
The uncharged t-RNA moves to E-site and then out of ribsome.
A dipeptide t RNA is formed on A site.
After the formation of peptide bond ,the tRNA at ‘P’ site is
deacylated & the tRNA at ‘A’ site now carries the polypeptide.

Translocation
 As the peptide bond formation occurs the ribosome moves to
the next condon of the mRNA towards its 3’-end ( i.e., forn 1st
to 2nd condon and 2nd to 3rd condon ) This process called
Translocation.
 Since the dipeptidyl tRNA is still attached to second codon, the
movement of ribosome shifts the dipeptidyl tRNA from A site to
Psite.
 The ribosome with its attached dipeptidyl tRNA & mRNA is
ready for another elongation cycle to attach the third amino
acid.

As the risosome moves from codon to codon along the mRNA
towards its 3’end, the polypeptide chain of the last amino acid is to
be inserted.
The role of the small subunit during elongation is one of
“decoding” the triplets present in mRNA, while role of the large
subunit is peptide-bond synthesis.

Termination
Termination, the third phase of translation.
Termination of protein synthesis is signalled by one or more of
three triplet codes in A site :- UAG, UAA, UGA.
These codons do not specify an amino acid, nor do they call for a
tRNA in the A site.
These codons are called stop codons termination codons or
nonsense codons.
The termination codons signals the action of GTP- dependent
release factors, which cleave the polypeptide chain from the
terminal tRNA releasing it from the translation complex.
Once this cleavage occurs, the tRNA is released from the ribosome,
which then dissociates in to its subunits.

Termination of polypeptide chain

MODIFICATION OF RELEASE POLYPEPTIDE
The just released polypeptide has primary structure i.e.; it is a
straight linear molecule.
It may lose some amino acids from the end with the help of an
exopeptidase enzymes, and then coil and fold on itself to acquire
secondary and tertiory structure. It may combine with other
polypeptides to have quaternary structure.
The protein synthisized on free polysomes are released in to the
cytoplasm and function as structural and enzymatic protein.
The proteins formed on the polysomes attached to ER pass into the
ER channels and are exported as cells secretions by exocytosis after
packaging in the golgi apparatus.

CONCLUSION
Translation is the process in which a polypeptide chain obtained of
different amino acids from the base sequence of an mRNA molecule
in association with a ribosome.
In other word “it is process by which genetic information
present in mRNA are translated into the language of Protein”
Translation is much more complex than transcription as it involves
initiation,elongation and termination of polypeptide chains.

REFERENCE
BOOK NAME EDITION WRITER
Molecular biology of the
gene
5th edition Watson
Baker
Bell
Cell & Molecular
Biology
6th edition Gerald Karp
INTERNET TIME
www.bioweb.wku.edu.com 4/03/2013 6:30pm
www.wikipedia.com 5/03/2013 9:30pm
www.bass.bio.uci.edu.com 4/03/2013 7:00pm

prokaryotic translation mechinry

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prokaryotic translation mechinry